Cool thing is you can program the teensy to be any usb device you want. You could plug it into your computer and make it act as a midi interface (with built in NJM in this case) AND have a midi port attached to it for standalone control. On top of that you can connect pots and switches to the board.
There's already a good library for handling MIDI messages which would probably make your project a lot easier. Check them out, there are different Teensy flavours (CPU power and i/o ports differ) (but all are not expensive at all).