@shift I don't have dedicated threads for each individual lr-mess system I run, but the short answer is "more-or-less all of them".
Just to rattle off some of the ones I've tried on my Pi, with decent-to-perfect performance:
Epoch Super Cassette Vision
Various plug-and-play systems (both "official" and bootlegs)
Fujitsu FM Towns
Once you get down how the core works (which admittedly is a bit of a nightmare), it's insanely versatile since it runs all the same systems as modern standalone MAME (according to the current docs, that's roughly 32,000; that link at the top is a good few years out of date). Arguably the most powerful core in all of RetroPie.
Is there an lr-mame core? Could I build one? I have used lr-mess both prebuilt and built from source and I see from its build script that it just pulls from https://github.com/libretro/mame.git.
Yes. Both lr-mess and lr-mame are built from the same codebase you mentioned (forked upstream MAME), but with different build options, which results in different supported systems (arcade vs. non-arcade).
I was referring to MAME (the upstream project), not the libretro cores.
Does this mean that mame itself is the part of the mame distribution that can run that system?
Upstream MAME distributes only 1 binary which supports all systems included - either arcade, computer, mechanical, etc. There's no separate MESS anymore and I think hasn't been for quite some time.
Your pointer was half of the solution so let me write the other half in case anyone else searches for this answer.
Setting the resolution in the retroarch Options menu did not have any effect until I also turned on "Alternate render method" in the same menu. At that point all resolution changes took immediate effect (upon leaving the retroarch UI) and now this all works perfectly.
I will write up the rest of my method to launch rom-less system soon!
I am going to answer this one myself in case anyone else needs this info in the future. After looking through the run_mess.sh script itself, I was able to see that the message "hn613128pc64.bin NOT FOUND (tried in pv2000)" was trying to tell me that lr-mess was looking for the BIOS file in a pv2000 subdirectory of the main BIOS directory. Once it was placed there the logic to skip the software list/hash started working properly and I was able to launch roms that did not match the official hashes.
Thanks for the fps check.
For the sound, It depends, for some 2D games, the sound is not that bad (not far from a good experience).
In the Hash file , located in the BIOS/mame/hash folder , they commented slightly on the playability.
Some games are not playables even on both emulators.
I'm running Zool 2 or Bubsy at 49 FPS with Lr-Mess and 33 FPS with Lr- virtualjaguar (Big difference)