Realizing

General

I tested an USB (-Serial) connection between an Arduino and the MiSTer (not MiSTer's User Port (Serial IO))
as this is an standard communication feature on Arduino's and easy to use.
And USB Ports are available for most of the MiSTer users is some way.
My ATMega 1284 with an FTDI Serial Adapter was sucessfully detected as /dev/ttyUSB0 by the MiSTer.
Later I used an ESP32 with an CP21xx Chip because of storage and speed.
My actual Display is an 2.8" SSD1322-OLED with 256x64 Pixel
connected via SPI to the TTGO-T8 v1.7.1 ESP32 MCU.

Arduino

For the Arduino Code you need to add some Libraries to your Arduino System.
See https://github.com/venice1200/MiSTer_tty2oled/wiki/Arduino-Settings for more Details.
The Arduino Code can be easily adapted to other Displays, Display-Libraries or Arduino's.

MiSTer

On the MiSTer you need two scripts and the INI Files tty2oled-user.ini and tty2oled-system.ini.
The first script S60tty2oled (in /media/fat/tty2oled/) is an starter script
for the Daemon script tty2oled.sh (in /media/fat/tty2oled/) which handles
the communication to the Arduino via the USB-Serial tty device.
After copying the two scripts to the MiSTer make them executable using chmod +x [scriptname].
The ini files must be copied to /media/fat/tty2oled/.

Pictures

tty2oled need pictures in XBM (B/W) or GSC (16 Greyscale Level) Format.
XBM's can be converted using Gimp (Export), Krita or ImageMagick magick convert <source.ext> <target.xbm>.
For creating GSC's see here https://github.com/venice1200/MiSTer_tty2oled/wiki/Build_GSC_Gimp .
The included XBM and GSC Pictures are converted to 256x64 Pixel.

Installation

The very easiest way to do the tty2oled setup on your MiSTer is to use the "Updater".
Go to MiSTer Installation & Update.