Just letting you know, the current .dll in your Genesis Plus GX Cyclone repo is broken, I think it didn't compile correctly.

If it helps with figuring out where the header for those special "AtGames/Firecore only" games comes from, it definitely comes from (at least a prototype of) the homebrew game "Mighty Mighty Missile!" for the Genesis. I was able to confirm it by looking at the game's TCRF page: https://tcrf.net/Proto:Mighty_Mighty_Missile! Really, it's just the fact that "FVRING" is mentioned right there on the game's title screen.

I found another bug with your Genesis Plus GX fork. When you beat level 2 of Plumbing Contest, the game breaks.

Your fork of Genesis Plus GX is able to emulate the dumped D-CAT16 handheld (it's the first emulator that can do so, actually!) however it boots up none of the games. I can tell they're (probably) there, mostly just because the original PCM music is there when you open the file as an uncompressed 8-bit PCM file in Audacity. Maybe you could look into this and see if you could possibly get them to run in the emulator?