The code is available (Java) from the SVN repositiory on the Google Code site. I used NetBeans to write it and then wrapped it in an OS X app wrapper with one of Apple's Developer tools.
The serial port speed seems to be the big problem with MIDI output directly from an Arduino. I ran into that limitation and decided to do the software route to solve the problem.
But if your instrument is already outputting MIDI, why not just get a MIDI interface for it? Cleaner...easier. I tried a single channel M-Audio UNO that worked fine...it was only $40 from Guitar Center.
That said, if you can read the data at the correct serial speed and parse the MIDI properly in Java, I don't see why the code couldn't be made to do exactly what you want.