Hi,
I modified ep128emu a bit to be able to work as a libretro core, with RetroArch frontend. Most things seem to work, and a few convenience items are also added like content autostart (where possible).

Find it here: https://github.com/zoltanvb/ep128emu-core/releases/tag/core_v0.7 . Currently Linux and ARM versions are downloadable.

Comments welcome!

v0.85 is out, with smaller improvements:
https://github.com/zoltanvb/ep128emu-core/releases/tag/core_v0.85

v0.90 is out, with smaller improvements mostly related to configuration options.
https://github.com/zoltanvb/ep128emu-core/releases/tag/core_v0.90

v0.93 is available, small improvements like speaker state control.
https://github.com/zoltanvb/ep128emu-core/releases/tag/core_v0.93

It is now available from inside RetroArch, as a downloadable core, on Windows and Linux x86 platforms. Enable experimental cores, and then it should appear.

Now the armv7 version is also available via the online updater: https://zoltanvb.github.io/armv7-hf-neon/ .
To use it, update retroarch.cfg to contain this:
core_updater_buildbot_cores_url = "https://zoltanvb.github.io/armv7-hf-neon/"
Afterwards, core downloader will find it. ROM files are still needed.

Also, MacOS builds are available since a while (that one is present in the default buildbot, no config file editing needed).

v1.2.0 is available. Now memory is exposed and you can use Retroarch's built-in memory analyzer to set up cheats/trainers: https://docs.libretro.com/guides/cheat-codes/#retroarch-new-cheat-code-searching

v1.2.6 is available. Minor improvements recently, mostly regarding multi-disk / multi-tape game handling, and LED driver is added. To emulate floppy activity LED with keyboard scroll lock, add to retroarch.cfg:
led_driver = "keyboard"
led1_map = "-1"
led2_map = "-1"
led3_map = "2"
There are a few "animated" RetroArch overlays around, it can be used also with those:
led_driver = "overlay"
led1_map = "1"
led2_map = "2"