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?
There is SoftMPU, which is a TSR emulating a MPU-401 and redirecting it to the SoundBlaster MIDI ports.
Not exactly what you want, but if you somehow manage to attach a "play MIDI using OPL2 or OPL3" library to it (if CPU speed is fast enough to permit that), it might do what you want.
At least you can use it as a blueprint on how to intercept MPU-401 accesses.