Consider implementing an interface for VSTi plugins for MIDI playback.

VST is Steinberg's format for audio plugins and is widely used in most professional software. VSTi is the virtual instrument specific version of that. The idea being someone writes their plugin as a DLL (or other sort of library for other platforms) and it can then load in any host that supports VST.

Works extremely well in the professional audio world.

The benefit to DOSBox would be twofold:

1) Any professional GM synth out there could be loaded. Something like Native Instrument's Bandstand or the like. Users could plug in whatever synth they choose to purchase. Obviously if they load a non-GM synth it won't work well, but that isn't something you have to worry about. Your program simply loads the VSTi and sends it MIDI data. What it does with it is its own business.

2) It would provide an easy interface for developers who wish to make MIDI engines for your software. For example the MT-32 synth could be made as a VST instead of any sort of integration. That means they can be updated out of cycle with your program, and distributed separately.

I do not know all the details of what is involved with VST support, but it has a full SDK out there http://ygrabit.steinberg.de/~ygrabit/public_html/index.html. It looks to be a BSD style license according to Sourceforge.

Just something to look at. Might make life easier in terms of having accurate General MIDI and MT-32 playback, what with the dearth of GM hardware these days.