diff --git a/CHANGELOG.md b/CHANGELOG.md
index ad4ce1ed7..f235bfab4 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,19 @@
+# tectonic 0.2.0 (2020-10-21)
+
+The 0.2 series finally adds "unstable" `-Z` flags! These allow you to configure
+engine options that are relatively low-level. The hope is to eventually set
+these kinds of things in a `Tectonic.toml` file, so their use is mildly
+discouraged, and long-term availability is not guaranteed. But there are plenty
+of times when such flags can be helpful. The currently supported options are:
+
+- `-Z min-crossrefs=<num>` controls the `-min-crossrefs` option of standalone `bibtex`
+- `-Z paper-size=<spec>` lets you control the output paper size, rather than
+  hardcoding it to be US Letter.
+
+Enormous thanks to @ralismark for finally implementing this! (#657) Now that the
+infrastructure is in place, suggestions for additional flags are more than
+welcome.
+
 # tectonic 0.1.17 (2020-10-13)
 
 - Fix source-based installation by updating to cbindgen 0.15, after later
diff --git a/Cargo.lock b/Cargo.lock
index f49637a06..99954d0a6 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -1126,9 +1126,9 @@ checksum = "d4fd5641d01c8f18a23da7b6fe29298ff4b55afcccdf78973b24cf3175fee32e"
 
 [[package]]
 name = "pkg-config"
-version = "0.3.18"
+version = "0.3.19"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "d36492546b6af1463394d46f0c834346f31548646f6ba10849802c9c9a27ac33"
+checksum = "3831453b3449ceb48b6d9c7ad7c96d5ea673e9b470a1dc578c2ce6521230884c"
 
 [[package]]
 name = "ppv-lite86"
@@ -1509,18 +1509,18 @@ checksum = "388a1df253eca08550bef6c72392cfe7c30914bf41df5269b68cbd6ff8f570a3"
 
 [[package]]
 name = "serde"
-version = "1.0.116"
+version = "1.0.117"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "96fe57af81d28386a513cbc6858332abc6117cfdb5999647c6444b8f43a370a5"
+checksum = "b88fa983de7720629c9387e9f517353ed404164b1e482c970a90c1a4aaf7dc1a"
 dependencies = [
  "serde_derive",
 ]
 
 [[package]]
 name = "serde_derive"
-version = "1.0.116"
+version = "1.0.117"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "f630a6370fd8e457873b4bd2ffdae75408bc291ba72be773772a4c2a065d9ae8"
+checksum = "cbd1ae72adb44aab48f325a02444a5fc079349a8d804c1fc922aed3f7454c74e"
 dependencies = [
  "proc-macro2",
  "quote",
@@ -1607,9 +1607,9 @@ checksum = "8ea5119cdb4c55b55d432abb513a0429384878c15dde60cc77b1c99de1a95a6a"
 
 [[package]]
 name = "structopt"
-version = "0.3.19"
+version = "0.3.20"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "a7a7159e7d0dbcab6f9c980d7971ef50f3ff5753081461eeda120d5974a4ee95"
+checksum = "126d630294ec449fae0b16f964e35bf3c74f940da9dca17ee9b905f7b3112eb8"
 dependencies = [
  "clap",
  "lazy_static",
@@ -1618,9 +1618,9 @@ dependencies = [
 
 [[package]]
 name = "structopt-derive"
-version = "0.4.12"
+version = "0.4.13"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "8fc47de4dfba76248d1e9169ccff240eea2a4dc1e34e309b95b2393109b4b383"
+checksum = "65e51c492f9e23a220534971ff5afc14037289de430e3c83f9daf6a1b6ae91e8"
 dependencies = [
  "heck",
  "proc-macro-error",
@@ -1654,7 +1654,7 @@ dependencies = [
 
 [[package]]
 name = "tectonic"
-version = "0.1.17"
+version = "0.2.0"
 dependencies = [
  "app_dirs2",
  "atty",
@@ -1689,14 +1689,14 @@ dependencies = [
 
 [[package]]
 name = "tectonic_cfg_support"
-version = "0.1.0"
+version = "0.1.1"
 dependencies = [
  "lazy_static",
 ]
 
 [[package]]
 name = "tectonic_xdv"
-version = "0.1.9"
+version = "0.1.10"
 dependencies = [
  "byteorder",
  "clap",
diff --git a/Cargo.toml b/Cargo.toml
index fdaf48048..01f18756f 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -4,7 +4,7 @@
 
 [package]
 name = "tectonic"
-version = "0.1.17"
+version = "0.2.0"
 authors = ["Peter Williams <peter@newton.cx>"]
 build = "build.rs"
 description = """
diff --git a/cfg_support/CHANGELOG.md b/cfg_support/CHANGELOG.md
index c27274410..a36c43fe6 100644
--- a/cfg_support/CHANGELOG.md
+++ b/cfg_support/CHANGELOG.md
@@ -1,8 +1,4 @@
-# See elsewhere for changelog
+# tectonic_cfg_support 0.1.1 (2020-10-21)
 
-This project’s release notes are curated from the Git history of its main
-branch. You can find them by looking at [the version of this file on the
-`release` branch][branch] or the [GitHub release history][gh-releases].
-
-[branch]: https://github.com/tectonic-typesetting/tectonic/blob/release/cfg_support/CHANGELOG.md
-[gh-releases]: https://github.com/tectonic-typesetting/tectonic/releases
+- No code changes; just issuing a new release to update deps and silence
+  Cranko's change detection.
diff --git a/cfg_support/Cargo.toml b/cfg_support/Cargo.toml
index 27be76b66..8f2af7dac 100644
--- a/cfg_support/Cargo.toml
+++ b/cfg_support/Cargo.toml
@@ -3,7 +3,7 @@
 
 [package]
 name = "tectonic_cfg_support"
-version = "0.1.0"
+version = "0.1.1"
 authors = [
     "Matt Rice <ratmice@gmail.com>",
     "Peter Williams <peter@newton.cx>"
diff --git a/src/bin/tectonic.rs b/src/bin/tectonic.rs
index fc301b143..927764264 100644
--- a/src/bin/tectonic.rs
+++ b/src/bin/tectonic.rs
@@ -16,6 +16,7 @@ use tectonic::errors::{ErrorKind, Result};
 use tectonic::status::plain::PlainStatusBackend;
 use tectonic::status::termcolor::TermcolorStatusBackend;
 use tectonic::status::{ChatterLevel, StatusBackend};
+use tectonic::unstable_opts::{UnstableArg, UnstableOptions};
 use tectonic::{errmsg, tt_error, tt_note};
 
 #[derive(Debug, StructOpt)]
@@ -73,11 +74,18 @@ struct CliOptions {
     /// The directory in which to place output files [default: the directory containing <input>]
     #[structopt(name = "outdir", short, long, parse(from_os_str))]
     outdir: Option<PathBuf>,
+    /// Unstable options. Pass -Zhelp to show a list
+    // TODO we can't pass -Zhelp without also passing <input>
+    #[structopt(name = "option", short = "Z", number_of_values = 1)]
+    unstable: Vec<UnstableArg>,
 }
 fn inner(args: CliOptions, config: PersistentConfig, status: &mut dyn StatusBackend) -> Result<()> {
+    let unstable = UnstableOptions::from_unstable_args(args.unstable.into_iter());
+
     let mut sess_builder = ProcessingSessionBuilder::default();
     let format_path = args.format;
     sess_builder
+        .unstables(unstable)
         .format_name(&format_path)
         .keep_logs(args.keep_logs)
         .keep_intermediates(args.keep_intermediates)
diff --git a/src/driver.rs b/src/driver.rs
index 0f53bf3ec..4e7c45598 100644
--- a/src/driver.rs
+++ b/src/driver.rs
@@ -26,6 +26,7 @@ use crate::engines::IoEventBackend;
 use crate::errors::{ErrorKind, Result, ResultExt};
 use crate::io::{Bundle, InputOrigin, IoProvider, IoSetup, IoSetupBuilder, OpenResult};
 use crate::status::StatusBackend;
+use crate::unstable_opts::UnstableOptions;
 use crate::{ctry, errmsg, tt_error, tt_note, tt_warning};
 use crate::{BibtexEngine, Spx2HtmlEngine, TexEngine, TexResult, XdvipdfmxEngine};
 use std::result::Result as StdResult;
@@ -326,6 +327,7 @@ pub struct ProcessingSessionBuilder {
     keep_logs: bool,
     synctex: bool,
     build_date: Option<SystemTime>,
+    unstables: UnstableOptions,
 }
 
 impl ProcessingSessionBuilder {
@@ -471,6 +473,12 @@ impl ProcessingSessionBuilder {
         self
     }
 
+    /// Loads unstable options into the processing session
+    pub fn unstables(&mut self, opts: UnstableOptions) -> &mut Self {
+        self.unstables = opts;
+        self
+    }
+
     /// Creates a `ProcessingSession`.
     pub fn create(self, status: &mut dyn StatusBackend) -> Result<ProcessingSession> {
         let mut io = IoSetupBuilder::default();
@@ -558,6 +566,7 @@ impl ProcessingSessionBuilder {
             keep_logs: self.keep_logs,
             synctex_enabled: self.synctex,
             build_date: self.build_date.unwrap_or(SystemTime::UNIX_EPOCH),
+            unstables: self.unstables,
         })
     }
 }
@@ -622,6 +631,8 @@ pub struct ProcessingSession {
 
     /// See `TexEngine::with_date` and `XdvipdfmxEngine::with_date`.
     build_date: SystemTime,
+
+    unstables: UnstableOptions,
 }
 
 const DEFAULT_MAX_TEX_PASSES: usize = 6;
@@ -1040,7 +1051,14 @@ impl ProcessingSession {
             TexEngine::new()
                 .halt_on_error_mode(true)
                 .initex_mode(true)
-                .process(&mut stack, &mut self.events, status, "UNUSED.fmt", "texput")
+                .process(
+                    &mut stack,
+                    &mut self.events,
+                    status,
+                    "UNUSED.fmt",
+                    "texput",
+                    &self.unstables,
+                )
         };
 
         match result {
@@ -1110,6 +1128,7 @@ impl ProcessingSession {
                     status,
                     &self.format_name,
                     &self.primary_input_tex_path,
+                    &self.unstables,
                 )
         };
 
@@ -1137,6 +1156,7 @@ impl ProcessingSession {
                 &mut self.events,
                 status,
                 &self.tex_aux_path.to_str().unwrap(),
+                &self.unstables,
             )
         };
 
@@ -1174,6 +1194,7 @@ impl ProcessingSession {
                 status,
                 &self.tex_xdv_path.to_str().unwrap(),
                 &self.tex_pdf_path.to_str().unwrap(),
+                &self.unstables,
             )?;
         }
 
diff --git a/src/engines/bibtex.rs b/src/engines/bibtex.rs
index 32cc64b8f..b560f1968 100644
--- a/src/engines/bibtex.rs
+++ b/src/engines/bibtex.rs
@@ -9,6 +9,12 @@ use super::{ExecutionState, IoEventBackend, TectonicBridgeApi};
 use crate::errors::{ErrorKind, Result};
 use crate::io::IoStack;
 use crate::status::StatusBackend;
+use crate::unstable_opts::UnstableOptions;
+
+#[repr(C)]
+pub struct BibtexConfig {
+    min_crossrefs: i32,
+}
 
 #[derive(Default)]
 pub struct BibtexEngine {}
@@ -24,6 +30,7 @@ impl BibtexEngine {
         events: &mut dyn IoEventBackend,
         status: &mut dyn StatusBackend,
         aux: &str,
+        unstables: &UnstableOptions,
     ) -> Result<TexResult> {
         let _guard = super::ENGINE_LOCK.lock().unwrap(); // until we're thread-safe ...
 
@@ -31,9 +38,12 @@ impl BibtexEngine {
 
         let mut state = ExecutionState::new(io, events, status);
         let bridge = TectonicBridgeApi::new(&mut state);
+        let config = BibtexConfig {
+            min_crossrefs: unstables.min_crossrefs.unwrap_or(2),
+        };
 
         unsafe {
-            match super::bibtex_simple_main(&bridge, caux.as_ptr()) {
+            match super::bibtex_simple_main(&bridge, &config, caux.as_ptr()) {
                 0 => Ok(TexResult::Spotless),
                 1 => Ok(TexResult::Warnings),
                 2 => Ok(TexResult::Errors),
diff --git a/src/engines/mod.rs b/src/engines/mod.rs
index 82e766711..842b1c1b2 100644
--- a/src/engines/mod.rs
+++ b/src/engines/mod.rs
@@ -496,6 +496,7 @@ extern "C" {
 
     fn dvipdfmx_simple_main(
         api: &TectonicBridgeApi,
+        config: &xdvipdfmx::XdvipdfmxConfig,
         dviname: *const libc::c_char,
         pdfname: *const libc::c_char,
         enable_compression: bool,
@@ -505,6 +506,7 @@ extern "C" {
 
     fn bibtex_simple_main(
         api: &TectonicBridgeApi,
+        config: &bibtex::BibtexConfig,
         aux_file_name: *const libc::c_char,
     ) -> libc::c_int;
 
diff --git a/src/engines/tex.rs b/src/engines/tex.rs
index f55d896e9..d9bc06f3f 100644
--- a/src/engines/tex.rs
+++ b/src/engines/tex.rs
@@ -9,6 +9,7 @@ use super::{ExecutionState, IoEventBackend, TectonicBridgeApi};
 use crate::errors::{DefinitelySame, ErrorKind, Result};
 use crate::io::IoStack;
 use crate::status::StatusBackend;
+use crate::unstable_opts::UnstableOptions;
 
 #[derive(Clone, Copy, Debug, Eq, PartialEq)]
 pub enum TexResult {
@@ -109,6 +110,7 @@ impl TexEngine {
         status: &mut dyn StatusBackend,
         format_file_name: &str,
         input_file_name: &str,
+        unstables: &UnstableOptions,
     ) -> Result<TexResult> {
         let _guard = super::ENGINE_LOCK.lock().unwrap(); // until we're thread-safe ...
 
@@ -119,6 +121,11 @@ impl TexEngine {
         let bridge = TectonicBridgeApi::new(&mut state);
 
         // initialize globals
+
+        let v = if unstables.shell_escape { 1 } else { 0 };
+        unsafe {
+            super::tt_xetex_set_int_variable(b"shell_escape_enabled\0".as_ptr() as _, v);
+        }
         let v = if self.halt_on_error { 1 } else { 0 };
         unsafe {
             super::tt_xetex_set_int_variable(b"halt_on_error_p\0".as_ptr() as _, v);
diff --git a/src/engines/xdvipdfmx.rs b/src/engines/xdvipdfmx.rs
index 45ab2a686..c7b9d110d 100644
--- a/src/engines/xdvipdfmx.rs
+++ b/src/engines/xdvipdfmx.rs
@@ -9,6 +9,12 @@ use super::{ExecutionState, IoEventBackend, TectonicBridgeApi};
 use crate::errors::{ErrorKind, Result};
 use crate::io::IoStack;
 use crate::status::StatusBackend;
+use crate::unstable_opts::UnstableOptions;
+
+#[repr(C)]
+pub struct XdvipdfmxConfig {
+    paperspec: *const libc::c_char,
+}
 
 pub struct XdvipdfmxEngine {
     enable_compression: bool,
@@ -51,9 +57,29 @@ impl XdvipdfmxEngine {
         status: &mut dyn StatusBackend,
         dvi: &str,
         pdf: &str,
+        unstables: &UnstableOptions,
     ) -> Result<i32> {
         let _guard = super::ENGINE_LOCK.lock().unwrap(); // until we're thread-safe ...
 
+        // This conversion is probably way too complex, because we need to convert String to
+        // something which holds a CStr (which needs to be a local so it doesn't disappear). And
+        // all of this happens in an Option.
+
+        // Keep a local reference so the string doesn't get dropped too early
+        let paperspec_str = unstables
+            .paper_size
+            .as_ref()
+            .and_then(|s| CString::new(s.clone()).ok());
+
+        // We default to "letter" paper size by default
+        let paperspec_default = CStr::from_bytes_with_nul(b"letter\0").unwrap();
+
+        let config = XdvipdfmxConfig {
+            paperspec: paperspec_str
+                .as_ref()
+                .map_or(paperspec_default.as_ptr(), |s| s.as_ptr()),
+        };
+
         let cdvi = CString::new(dvi)?;
         let cpdf = CString::new(pdf)?;
 
@@ -63,6 +89,7 @@ impl XdvipdfmxEngine {
         unsafe {
             match super::dvipdfmx_simple_main(
                 &bridge,
+                &config,
                 cdvi.as_ptr(),
                 cpdf.as_ptr(),
                 self.enable_compression,
diff --git a/src/io/mod.rs b/src/io/mod.rs
index c2e3a299d..c740125d8 100644
--- a/src/io/mod.rs
+++ b/src/io/mod.rs
@@ -312,11 +312,7 @@ impl<T> OpenResult<T> {
 
     /// Returns true if this result is of the NotAvailable variant.
     pub fn is_not_available(&self) -> bool {
-        if let OpenResult::NotAvailable = *self {
-            true
-        } else {
-            false
-        }
+        matches!(*self, OpenResult::NotAvailable)
     }
 
     /// Convert this object into a plain Result, erroring if the item was not available.
@@ -471,7 +467,7 @@ pub trait Bundle: IoProvider {
             OpenResult::NotAvailable => {
                 // Broken or un-cacheable backend.
                 return Err(ErrorKind::Msg(
-                    "itar-format bundle does not provide needed SHA256SUM file".to_owned(),
+                    "bundle does not provide needed SHA256SUM file".to_owned(),
                 )
                 .into());
             }
diff --git a/src/lib.rs b/src/lib.rs
index 5954d43e2..9aa809c64 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -56,6 +56,7 @@ pub mod engines;
 pub mod errors;
 pub mod io;
 pub mod status;
+pub mod unstable_opts;
 
 // Note: this module is intentionally *not* gated by #[cfg(test)] -- see its
 // docstring for details.
diff --git a/src/unstable_opts.rs b/src/unstable_opts.rs
new file mode 100644
index 000000000..c5a5be14e
--- /dev/null
+++ b/src/unstable_opts.rs
@@ -0,0 +1,101 @@
+// src/bin/tectonic.rs -- Command-line driver for the Tectonic engine.
+// Copyright 2020 the Tectonic Project
+// Licensed under the MIT License.
+
+//! Unstable options for the Tectonic engine.
+//!
+//! This is similar to the -Z options on rustc - they're unstable options that are not guaranteed
+//! to be reliable or very polished. In particular, many of these prevent the build from being
+//! reproducible.
+
+use crate::errors::{Error, Result};
+use std::default::Default;
+use std::str::FromStr;
+
+const HELPMSG: &str = r#"Available unstable options:
+
+    -Z help                     Lists all unstable options
+    -Z min-crossrefs=<num>      Equivalent to bibtex's -min-crossrefs flag - "include after <num>
+                                    crossrefs" [default: 2]
+    -Z paper-size=<spec>        Change the default paper size [default: letter]
+"#;
+
+// Re-add this when \write18 gets implemented
+//    -Z shell-escape             Enable \write18
+
+// Each entry of this should correspond to a field of UnstableOptions.
+#[derive(Debug)]
+pub enum UnstableArg {
+    Help,
+    PaperSize(String),
+    MinCrossrefs(i32),
+    ShellEscapeEnabled,
+}
+
+impl FromStr for UnstableArg {
+    type Err = Error;
+
+    /// Parse from the argument to -Z
+    fn from_str(s: &str) -> Result<Self> {
+        let mut splitter = s.splitn(2, '=');
+        let arg = splitter.next().unwrap(); // splitn will always have at least 1 item
+        let value = splitter.next();
+
+        // For structopt/clap, if you pass a value to a flag which doesn't accept one, it's
+        // silently ignored.
+
+        match arg {
+            "help" => Ok(UnstableArg::Help),
+
+            "min-crossrefs" => value
+                .ok_or_else(|| {
+                    "'-Z min-crossrefs <spec>' requires a value but none was supplied".into()
+                })
+                .and_then(|s| {
+                    FromStr::from_str(s).map_err(|e| format!("-Z min-crossrefs: {}", e).into())
+                })
+                .map(UnstableArg::MinCrossrefs),
+
+            "paper-size" => value
+                .ok_or_else(|| {
+                    "'-Z paper-size <spec>' requires a value but none was supplied".into()
+                })
+                .map(|s| UnstableArg::PaperSize(s.to_string())),
+
+            // Re-add this when \write18 gets implemetned
+            // "shell-escape" => Ok(UnstableArg::ShellEscapeEnabled),
+            _ => Err(format!("Unknown unstable option '{}'", arg).into()),
+        }
+    }
+}
+
+#[derive(Debug, Default)]
+pub struct UnstableOptions {
+    pub paper_size: Option<String>,
+    pub shell_escape: bool,
+    pub min_crossrefs: Option<i32>,
+}
+
+impl UnstableOptions {
+    pub fn from_unstable_args<I>(uargs: I) -> Self
+    where
+        I: Iterator<Item = UnstableArg>,
+    {
+        let mut opts = UnstableOptions::default();
+
+        for u in uargs {
+            use UnstableArg::*;
+            match u {
+                Help => {
+                    print!("{}", HELPMSG);
+                    std::process::exit(0);
+                }
+                MinCrossrefs(num) => opts.min_crossrefs = Some(num),
+                PaperSize(size) => opts.paper_size = Some(size),
+                ShellEscapeEnabled => opts.shell_escape = true,
+            }
+        }
+
+        opts
+    }
+}
diff --git a/tectonic/bibtex.c b/tectonic/bibtex.c
index 7c00b7335..aca94f58f 100644
--- a/tectonic/bibtex.c
+++ b/tectonic/bibtex.c
@@ -145,7 +145,6 @@ static jmp_buf error_jmpbuf, recover_jmpbuf;
 #define aux_stack_size 20
 #define MAX_BIB_FILES 20
 #define POOL_SIZE 65000L
-#define MIN_CROSSREFS 2
 #define MAX_STRINGS 35307
 #define MAX_CITES 750
 #define WIZ_FN_SPACE 3000
@@ -416,7 +415,8 @@ static buf_pointer num_text_chars;
 static unsigned char /*bad_conversion */ conversion_type;
 static bool prev_colon;
 static int verbose;
-static int32_t min_crossrefs;
+
+const BibtexConfig* bibtex_config;
 
 /*:473*//*12: *//*3: */
 
@@ -6695,7 +6695,7 @@ static void bst_read_command(void)
                                     mark_warning();
                                 }
                                 if (((!all_entries) && (cite_parent_ptr >= old_num_cites)
-                                     && (cite_info[cite_parent_ptr] < min_crossrefs)))
+                                     && (cite_info[cite_parent_ptr] < bibtex_config->min_crossrefs)))
                                     field_info[field_ptr] = 0 /*missing */ ;
                             }
                         }
@@ -6709,7 +6709,7 @@ static void bst_read_command(void)
 
                     if (type_list[cite_ptr] == 0 /*empty */ )
                         print_missing_entry(cite_list[cite_ptr]);
-                    else if ((all_entries) || (cite_ptr < old_num_cites) || (cite_info[cite_ptr] >= min_crossrefs)) {
+                    else if ((all_entries) || (cite_ptr < old_num_cites) || (cite_info[cite_ptr] >= bibtex_config->min_crossrefs)) {
                         if (cite_ptr > cite_xptr) {   /*286: */
                             if ((cite_xptr + 1) * num_fields > max_fields) {
                                 puts_log("field_info index is out of range");
@@ -7302,7 +7302,6 @@ bibtex_main(const char *aux_file_name)
     max_glob_strs = MAX_GLOB_STRS;
     max_fields = MAX_FIELDS;
     max_cites = MAX_CITES;
-    min_crossrefs = MIN_CROSSREFS;
     wiz_fn_space = WIZ_FN_SPACE;
     lit_stk_size = LIT_STK_SIZE;
 
diff --git a/tectonic/bibtex.h b/tectonic/bibtex.h
index 2a8851b4e..8df358cf3 100644
--- a/tectonic/bibtex.h
+++ b/tectonic/bibtex.h
@@ -10,4 +10,7 @@
 
 tt_history_t bibtex_main(const char *aux_file_name);
 
+// Passed in from rust side
+extern const BibtexConfig* bibtex_config;
+
 #endif
diff --git a/tectonic/core-bridge.c b/tectonic/core-bridge.c
index 992506754..a170fb9e0 100644
--- a/tectonic/core-bridge.c
+++ b/tectonic/core-bridge.c
@@ -70,7 +70,7 @@ tex_simple_main(const tt_bridge_api_t *api, const char *dump_name, const char *i
 
 
 int
-dvipdfmx_simple_main(const tt_bridge_api_t *api, const char *dviname, const char *pdfname, bool compress, bool deterministic_tags, time_t build_date)
+dvipdfmx_simple_main(const tt_bridge_api_t *api, const XdvipdfmxConfig* config, const char *dviname, const char *pdfname, bool compress, bool deterministic_tags, time_t build_date)
 {
     int rv;
 
@@ -81,6 +81,7 @@ dvipdfmx_simple_main(const tt_bridge_api_t *api, const char *dviname, const char
         return 99;
     }
 
+    dpx_config = config;
     rv = dvipdfmx_main(pdfname, dviname, NULL, 0, false, compress, deterministic_tags, false, 0, build_date);
     tectonic_global_bridge = NULL;
 
@@ -89,7 +90,7 @@ dvipdfmx_simple_main(const tt_bridge_api_t *api, const char *dviname, const char
 
 
 int
-bibtex_simple_main(const tt_bridge_api_t *api, const char *aux_file_name)
+bibtex_simple_main(const tt_bridge_api_t *api, const BibtexConfig *config, const char *aux_file_name)
 {
     int rv;
 
@@ -100,6 +101,7 @@ bibtex_simple_main(const tt_bridge_api_t *api, const char *aux_file_name)
         return 99;
     }
 
+    bibtex_config = config;
     rv = bibtex_main(aux_file_name);
     tectonic_global_bridge = NULL;
     return rv;
diff --git a/tectonic/dpx-dpxconf.h b/tectonic/dpx-dpxconf.h
index 0151c4c85..3edacd8f1 100644
--- a/tectonic/dpx-dpxconf.h
+++ b/tectonic/dpx-dpxconf.h
@@ -34,10 +34,6 @@ struct paper {
   double pswidth, psheight;
 };
 
-#ifndef DEFAULT_PAPER_NAME
-#define DEFAULT_PAPER_NAME "a4"
-#endif
-
 extern const struct paper  paperspecs[];
 const struct paper *paperinfo (const char *ppformat);
 
@@ -46,14 +42,9 @@ const struct paper *paperinfo (const char *ppformat);
 #define papername(p)       (((p) && (p)->name) ? p->name : NULL)
 #define paperfirst()       &(paperspecs[0])
 #define papernext(p)       ((((p)+1) && ((p)+1)->name) ? (p+1) : NULL)
-
-#define defaultpapername() DEFAULT_PAPER_NAME
-#define systempapername()  DEFAULT_PAPER_NAME
-
-#define defaultpapersizefile() NULL
-#define systempapersizefile()  NULL
 #endif /* HAVE_LIBPAPER */
 
 void dumppaperinfo (void);
+extern const char* paperspec;
 
 #endif /* _DPXCONF_H_ */
diff --git a/tectonic/dpx-dvipdfmx.c b/tectonic/dpx-dvipdfmx.c
index 6a1b6bc9a..e3398903c 100644
--- a/tectonic/dpx-dvipdfmx.c
+++ b/tectonic/dpx-dvipdfmx.c
@@ -63,6 +63,7 @@ typedef struct page_range
 
 int is_xdv = 0;
 int translate_origin = 0;
+const XdvipdfmxConfig* dpx_config;
 
 #define OPT_TPIC_TRANSPARENT_FILL (1 << 1)
 #define OPT_CIDFONT_FIXEDPITCH    (1 << 2)
@@ -263,16 +264,6 @@ select_pages (
   *ret_num_page_ranges = num_page_ranges;
 }
 
-static void
-system_default (void)
-{
-  if (systempapername() != NULL) {
-    select_paper(systempapername());
-  } else if (defaultpapername() != NULL) {
-    select_paper(defaultpapername());
-  }
-}
-
 #define SWAP(v1,v2) do {\
    double _tmp = (v1);\
    (v1) = (v2);\
@@ -411,8 +402,6 @@ dvipdfmx_main (
   pdf_set_compression(compress ? 9 : 0);
   pdf_font_set_deterministic_unique_tags(deterministic_tags ? 1 : 0);
 
-  system_default();
-
   pdf_init_fontmaps(); /* This must come before parsing options... */
 
   /* We used to read the config file here. It synthesized command-line
@@ -420,7 +409,7 @@ dvipdfmx_main (
    * code bits. */
 
   pdf_set_version (5);
-  select_paper("letter");
+  select_paper(dpx_config->paperspec);
   annot_grow = 0;
   bookmark_open = 0;
   key_bits = 40;
diff --git a/tectonic/dpx-dvipdfmx.h b/tectonic/dpx-dvipdfmx.h
index 25b88351c..784960069 100644
--- a/tectonic/dpx-dvipdfmx.h
+++ b/tectonic/dpx-dvipdfmx.h
@@ -34,6 +34,7 @@
 extern int is_xdv;
 extern int translate_origin;
 extern time_t source_date_epoch;
+extern const XdvipdfmxConfig* dpx_config;
 
 int extractbb(int argc, char *argv[]);
 int dvipdfmx_main(
diff --git a/tectonic/xetex-engine-interface.c b/tectonic/xetex-engine-interface.c
index 2aa23264f..1b1bde2d0 100644
--- a/tectonic/xetex-engine-interface.c
+++ b/tectonic/xetex-engine-interface.c
@@ -23,6 +23,8 @@ tt_xetex_set_int_variable (const char *var_name, int value)
         synctex_enabled = (value != 0);
     else if (streq_ptr(var_name, "semantic_pagination_enabled"))
         semantic_pagination_enabled = (value != 0);
+    else if (streq_ptr(var_name, "shell_escape_enabled"))
+        shell_escape_enabled = (value != 0);
     else
         return 1; /* Uh oh: unrecognized variable */
 
diff --git a/tectonic/xetex-ini.c b/tectonic/xetex-ini.c
index ca3484396..0bff9bdc7 100644
--- a/tectonic/xetex-ini.c
+++ b/tectonic/xetex-ini.c
@@ -9,6 +9,7 @@
 #include "dpx-pdfobj.h" /* pdf_files_{init,close} */
 
 /* All the following variables are declared in xetex-xetexd.h */
+bool shell_escape_enabled = false;
 memory_word *eqtb;
 int32_t bad;
 char *name_of_file;
diff --git a/tectonic/xetex-shipout.c b/tectonic/xetex-shipout.c
index 76bad8e59..88b9578c1 100644
--- a/tectonic/xetex-shipout.c
+++ b/tectonic/xetex-shipout.c
@@ -2174,15 +2174,31 @@ write_out(int32_t p)
         if (!log_opened)
             selector = SELECTOR_TERM_ONLY;
 
+        // Existing warning for shell-escape
+        diagnostic_begin_capture_warning_here();
+
         print_nl_cstr("runsystem(");
         for (d = 0; d <= (cur_length()) - 1; d++)
             print(str_pool[str_start[str_ptr - TOO_BIG_CHAR] + d]);
 
         print_cstr(")...");
-        print_cstr("disabled");
-        print_char('.');
+        if (!shell_escape_enabled) {
+            print_cstr("disabled");
+            print_char('.');
+        } else {
+            // Currently, -Z shell-escape is implemented but hidden (see
+            // src/unstable_opts.rs). When this gets actually implemented,
+            // uncomment the relevant parts in that file.
+
+            print_cstr("enabled but not implemented yet!");
+        }
+
+        capture_to_diagnostic(NULL);
+
         print_nl_cstr("");
         print_ln();
+
+        // Clear shell escape command
         pool_ptr = str_start[str_ptr - TOO_BIG_CHAR];
     }
 
diff --git a/tectonic/xetex-xetexd.h b/tectonic/xetex-xetexd.h
index af14c2ec7..c05ef8599 100644
--- a/tectonic/xetex-xetexd.h
+++ b/tectonic/xetex-xetexd.h
@@ -356,8 +356,8 @@ void remember_source_info(str_number, int);
 /* variables! */
 
 /* All the following variables are defined in xetexini.c */
+extern bool shell_escape_enabled;
 extern memory_word *eqtb;
-
 extern int32_t bad;
 extern char *name_of_file;
 extern UTF16_code *name_of_file16;
diff --git a/tests/bibtex.rs b/tests/bibtex.rs
index 7901763ee..1d3b8bd10 100644
--- a/tests/bibtex.rs
+++ b/tests/bibtex.rs
@@ -2,6 +2,7 @@
 // Licensed under the MIT License.
 
 use std::collections::HashSet;
+use std::default::Default;
 
 use tectonic::engines::NoopIoEventBackend;
 use tectonic::io::stdstreams::GenuineStdoutIo;
@@ -49,7 +50,13 @@ impl TestCase {
         let mut status = NoopStatusBackend::new();
 
         BibtexEngine::new()
-            .process(&mut io, &mut events, &mut status, &auxname)
+            .process(
+                &mut io,
+                &mut events,
+                &mut status,
+                &auxname,
+                &Default::default(),
+            )
             .unwrap();
 
         // Check that outputs match expectations.
diff --git a/tests/formats.rs b/tests/formats.rs
index 4f90fc9b6..87ed6161e 100644
--- a/tests/formats.rs
+++ b/tests/formats.rs
@@ -15,6 +15,7 @@
 /// to disk, which may be helpful in debugging. There is probably a less gross
 /// way to implement that option.
 use std::collections::{HashMap, HashSet};
+use std::default::Default;
 use std::ffi::{OsStr, OsString};
 use std::str::FromStr;
 
@@ -103,6 +104,7 @@ fn test_format_generation(texname: &str, fmtname: &str, sha256: &str) {
                 &mut NoopStatusBackend::new(),
                 "unused.fmt",
                 texname,
+                &Default::default(),
             )
             .unwrap();
     }
diff --git a/tests/tex-outputs.rs b/tests/tex-outputs.rs
index 3e9aea35b..019bc0139 100644
--- a/tests/tex-outputs.rs
+++ b/tests/tex-outputs.rs
@@ -11,6 +11,7 @@ use tectonic::errors::{DefinitelySame, ErrorKind, Result};
 use tectonic::io::testing::SingleInputFileIo;
 use tectonic::io::{FilesystemIo, FilesystemPrimaryInputIo, IoProvider, IoStack, MemoryIo};
 use tectonic::status::NoopStatusBackend;
+use tectonic::unstable_opts::UnstableOptions;
 use tectonic::{TexEngine, XdvipdfmxEngine};
 
 #[path = "util/mod.rs"]
@@ -23,6 +24,7 @@ struct TestCase {
     check_synctex: bool,
     check_pdf: bool,
     extra_io: Vec<Box<dyn IoProvider>>,
+    unstables: UnstableOptions,
 }
 
 impl TestCase {
@@ -33,6 +35,7 @@ impl TestCase {
             check_synctex: false,
             check_pdf: false,
             extra_io: Vec::new(),
+            unstables: UnstableOptions::default(),
         }
     }
 
@@ -56,6 +59,11 @@ impl TestCase {
         self
     }
 
+    fn with_unstables(&mut self, unstables: UnstableOptions) -> &mut Self {
+        self.unstables = unstables;
+        self
+    }
+
     fn expect(&mut self, result: Result<TexResult>) -> &mut Self {
         self.expected_result = result;
         self
@@ -112,8 +120,14 @@ impl TestCase {
             let mut events = NoopIoEventBackend::new();
             let mut status = NoopStatusBackend::new();
 
-            let tex_res =
-                TexEngine::new().process(&mut io, &mut events, &mut status, "plain.fmt", &texname);
+            let tex_res = TexEngine::new().process(
+                &mut io,
+                &mut events,
+                &mut status,
+                "plain.fmt",
+                &texname,
+                &self.unstables,
+            );
 
             if self.check_pdf && tex_res.definitely_same(&Ok(TexResult::Spotless)) {
                 XdvipdfmxEngine::new()
@@ -124,7 +138,14 @@ impl TestCase {
                             .checked_add(time::Duration::from_secs(1_456_304_492))
                             .unwrap(),
                     )
-                    .process(&mut io, &mut events, &mut status, &xdvname, &pdfname)
+                    .process(
+                        &mut io,
+                        &mut events,
+                        &mut status,
+                        &xdvname,
+                        &pdfname,
+                        &self.unstables,
+                    )
                     .unwrap();
             }
 
@@ -260,3 +281,13 @@ fn tectoniccodatokens_ok() {
 fn the_letter_a() {
     TestCase::new("the_letter_a").check_pdf(true).go()
 }
+
+#[test]
+fn a4paper() {
+    let mut unstables = UnstableOptions::default();
+    unstables.paper_size = Some(String::from("a4"));
+    TestCase::new("a4paper")
+        .with_unstables(unstables)
+        .check_pdf(true)
+        .go()
+}
diff --git a/tests/tex-outputs/a4paper.log b/tests/tex-outputs/a4paper.log
new file mode 100644
index 000000000..0f6bcbee3
--- /dev/null
+++ b/tests/tex-outputs/a4paper.log
@@ -0,0 +1,3 @@
+**
+(a4paper.tex [1] )
+Output written on a4paper.xdv (1 page, 212 bytes).
diff --git a/tests/tex-outputs/a4paper.pdf b/tests/tex-outputs/a4paper.pdf
new file mode 100644
index 000000000..3c7755632
Binary files /dev/null and b/tests/tex-outputs/a4paper.pdf differ
diff --git a/tests/tex-outputs/a4paper.tex b/tests/tex-outputs/a4paper.tex
new file mode 100644
index 000000000..7c9c44ad9
--- /dev/null
+++ b/tests/tex-outputs/a4paper.tex
@@ -0,0 +1 @@
+a\bye
diff --git a/tests/tex-outputs/a4paper.xdv b/tests/tex-outputs/a4paper.xdv
new file mode 100644
index 000000000..680f8afb5
Binary files /dev/null and b/tests/tex-outputs/a4paper.xdv differ
diff --git a/tests/trip.rs b/tests/trip.rs
index ca06e883d..dff4123d0 100644
--- a/tests/trip.rs
+++ b/tests/trip.rs
@@ -2,6 +2,7 @@
 // Copyright 2016-2018 the Tectonic Project
 // Licensed under the MIT License.
 
+use std::default::Default;
 /// Our incarnation of the classic TRIP test. Unfortunately, the test is
 /// defined in terms of the precise terminal output and error handling behavior
 /// of the engine, so you can't do anything to improve the (incredibly poor) UX
@@ -62,6 +63,7 @@ fn trip_test() {
                 &mut NoopStatusBackend::new(),
                 "INITEX",
                 "trip",
+                &Default::default(),
             )
             .unwrap();
     }
@@ -78,6 +80,7 @@ fn trip_test() {
                 &mut NoopStatusBackend::new(),
                 "trip.fmt",
                 "trip",
+                &Default::default(),
             )
             .unwrap();
     }
@@ -127,6 +130,7 @@ fn etrip_test() {
                 &mut NoopStatusBackend::new(),
                 "INITEX",
                 "etrip",
+                &Default::default(),
             )
             .unwrap();
     }
@@ -143,6 +147,7 @@ fn etrip_test() {
                 &mut NoopStatusBackend::new(),
                 "etrip.fmt",
                 "etrip",
+                &Default::default(),
             )
             .unwrap();
     }
diff --git a/tests/util/mod.rs b/tests/util/mod.rs
index 6974e5f2c..b737bf07d 100644
--- a/tests/util/mod.rs
+++ b/tests/util/mod.rs
@@ -13,6 +13,7 @@
 
 use ::flate2::read::GzDecoder;
 use std::collections::{HashMap, HashSet};
+use std::default::Default;
 use std::env;
 use std::ffi::{OsStr, OsString};
 use std::fs::File;
@@ -85,6 +86,7 @@ pub fn ensure_plain_format() -> Result<PathBuf> {
                     &mut NoopStatusBackend::new(),
                     "UNUSED.fmt",
                     "plain.tex",
+                    &Default::default(),
                 )?;
         }
 
diff --git a/xdv/CHANGELOG.md b/xdv/CHANGELOG.md
index 17b66fa53..2c39bf373 100644
--- a/xdv/CHANGELOG.md
+++ b/xdv/CHANGELOG.md
@@ -1,8 +1,4 @@
-# See elsewhere for changelog
+# tectonic_xdv 0.1.10 (2020-10-21)
 
-This project’s release notes are curated from the Git history of its main
-branch. You can find them by looking at [the version of this file on the
-`release` branch][branch] or the [GitHub release history][gh-releases].
-
-[branch]: https://github.com/tectonic-typesetting/tectonic/blob/release/xdv/CHANGELOG.md
-[gh-releases]: https://github.com/tectonic-typesetting/tectonic/releases
+- No code changes; just issuing a new release to update deps and silence
+  Cranko's change detection.
diff --git a/xdv/Cargo.toml b/xdv/Cargo.toml
index 2106da616..657d7cd9f 100644
--- a/xdv/Cargo.toml
+++ b/xdv/Cargo.toml
@@ -3,7 +3,7 @@
 
 [package]
 name = "tectonic_xdv"
-version = "0.1.9"
+version = "0.1.10"
 authors = ["Peter Williams <peter@newton.cx>"]
 description = """
 A decoder for the XDV and SPX file formats used by XeTeX and Tectonic.