Nobody replied to the last post, so I’m not sure if anyone is interested, but I’ll continue to post about my findings:
The only games I tried are the King’s Quest VGA remakes from AGD Interactive. My monitor resolution is 1024×768. If I used the –fullscreen option already in RetroPie’s emulators.cfg file, AGS would not be able to scale the image to fullscreen. Looking at the log file, I could see that it eventually resorted to windowed mode at 320×200. If I started in windowed mode, it would actually be able to scale 3x to 960×600. It isn’t fullscreen, but that’s the best I could manage with the launch options and editing the acsetup.cfg file.
I can see why this emulator is stuck in the experimental menu.