I found this project about using the ISA bus to talk to the FM chip in a soundcard. I have a box of old soundcards, however all of my soundcards have the OPL integrated in the main controller chip. So I found this project interesting. It should not be impossible to integrate a full ISA card in a midibox project, it would probably not be the prettiest box around.
"AdLib sound card on a parallel port"
ISA to parallel port adlib project
(I'm sorry if I'm reposting old knowledge here)