Hi there! Resurrecting this thread a couple years later
Is this still a possibility? It would be great to have XG instrument definitions at least, along with the Roland ones.
This site has a bunch of .midnams, including the Yamaha MU10 and MU128.
Alternately, might it be possible to enable an experimental/expert feature toggle, which would cause QMidi to look in the /Library/Audio/MIDI Patch Names directory for these files, rather than the internal one?