Use

get snap with zsync http://metadata.neon.kde.org/snap/plasma_5.9_amd64.snap.zsync
install sudo snap install --force-dangerous --devmode plasma_5.9_amd64.snap
this is only necessary when the snap was newly installed/reinstalled
- check snap list for installed rev of plasma snap (changes on each install). e.g. x8
- copy the bin/ directory from the repo into $HOME/snap/plasma/REVISION/
run (currently runs a bash) dbus-launch snap run plasma
setup environment $HOME/bin/envo
at this point startplasmacompositor would work etc.
simple test in xsession kwin_wayland --xwayland should bring up a black wayland window

Misc

the bin/ stuff is a hack so the snap doesn't have to rebuilt every time the environment needs to be expanded etc.
this uses a patched xwayland. xkb/ddxLoad.c in xorg is hardcompiling the xkbcomp path to /usr/bin, the patched build simply strips the pathing so xkbcomp is resolved from $PATH (nb: this is a security leak). There is a relocatibility feature resolving xkbcomp's location relative to the server binary, but it is only implemented and used in the windows server right now.
kinit hard-compile paths in start_kdeinit and start_kdeinit_wrapper. the entire pile of binaries seems a bit weird because in part is claims setuid is involved yet none of the involved binaries actually has the setuid set it seems. start_kdeinit does however oom_adj, whether or not that actually works is unknown given it has no setuid AND it seems to utterly ignore return values so I am guessing it probably does not work.
kinit has patches to turn the hard-compiled paths into lookup. for wrapper this lookup is relative within same-dir, for start_kdeinit (forks kdeinit) this needs to go through $PATH though. this would be a security hazard if it was setuid and passed that own to a fork but given it isn't to begin with this is probably also fine...