Have you tried just putting the serial communication at 31250 and send some basic MIDI commands. That could be one way of testing it. Sounds interesting that with the Roland driver, is it an emulator just forwarding serial data or is it a driver so that you see the device as a MIDI device?
Check out Tom Igoe's work, http://itp.nyu.edu/physcomp/Labs/MIDIOutput
. He has a MIDI jack connected directly to the serial pins of the board (not using the serial to USB), but then you need a MIDI input device on your computer. You should be able to see what data he is sending in the code though. Check out Tod's blog, http://todbot.com/blog/spookyarduino/
, also. Found some stuff there ones...