ChucK seems to support sending MIDI data.
As far as I understand, you dont want to modify SoundFonts, but just use them to play MIDI data, that you create or modify in your application ?
Here are the links for two MIDI drivers, that use the Soundfont-based software synthesizer inside the BASS Audio Libray:
2) BASSMIDI
Using the MIDI functions from WinMM.dll (MSDN) through ChucK's layer on top of it, you can play your MIDI data with SoundFonts.