There are a few games for MS-DOS with music only through MPU-401, like the platformer Abuse and the RPG Avalon.
Is there a TSR available that intercepts MPU-401 port accesses and transforms them into OPL2 or OPL3 music? I expect the result wouldn't be great, but I'd like to try. I think I could do this by running the games from within Windows 3.1 or 95, but is there a way in just DOS?