That isn't quite what I mean. What I'm not sure about is if games can run their OWN region check on the console (not the CD drive) independent of the CD BIOS.
In theory it'd allow things like changing language between English and Japanese, as all cartridge modes can including Master System and 32X.
I guess I could burn Sega Classics 4-in-1 or 5-in-1. To see if Revenge of Shinobi and Streets of Rage on those discs switch language depending whether I use my US Genesis or JP Megadrive attached to the Sega CD.