re. "their instrument banks were effectively hard-coded", the Creative Music System was a PSG-like chip with only square waves and no sample ROM at all, the AdLib was a FM synthesizer with no ROM banks either (though some MIDI banks which sounded nothing like sample-based MIDI players like the SC-55), and the GUS had a mix of ROM and RAM (and I don't think it targeted General MIDI either). I don't see any reason why any of these chips would be incompatible with trackers, and I do know the AdLib had multiple trackers historically and today (Reality Adlib Tracker, Adlib Tracker II, ScreamTracker 3, and more recently OpenMPT), though I think "feeding MIDIs through per-game custom soundbanks" was more common in video games, and AFAIK the GUS had more MIDI-based composing tools than trackers. Note that I don't know much about the CMS and GUS, and I have more experience with how Adlib music is composed/played today than in the 1990s.