Although the original hardware includes a PIC that is USB capable and has a serial programming breakout -- the original firmware is written in such a way that I think it will ultimately be easier to desolder the PIC and use the original pinout to create an interface for the teensy.

This would be a cleaner hack if I were to use the PIC18fxxxx chip that is on board, but not as easy to write the software. Software ease is trumping chip reuse.