I could not find a way to do this, there is only a Core Audio Midi output, which plays through the internal synthesizer or the Java synthesizer.
I'm developing a modeled analog synth for Mac OS X and I would like to route TuxGuitar Midi output to it, using MIDI Patchbay. The Midi device TuxGuitar uses for output should be configurable.
I guess you have difficulties to re-route your MIDI output in CoreAudio...
One thing is for sure, you can always write another MIDI plugin for TuxGuitar... take a look at TuxGuitar-CoreAudio plugin (project), there shouldn't be too many differences and you should easily connect it to your application, whatever the programming language you are using.