I thought about it. you need to use timidity 0.20i (the last one from the original author released under LGPL, the next ones are released under GPL). Then, to make it a real midi driver (in terms of Win32 API) which isn't a trivial task (for example, timidity pre-processes the full content of the MIDI file to be played, whilst a standard MIDI driver has to play midi notes on the fly...). I did start looking at that a long time ago, but without real results in so far.
I think it would be better to support ALSA midi and then people could use Timidity as an Alsa 'plug-in' to do a software sequencer.
Lionel