I confirm it works this way on 10.6.8, except that
1. I had to tick the "Device is online" option in the IAC Driver Bus window, in order to have MESS acknowledging it in -listmidi
2. I needed to select hijack in MidiPipe, because without it, I was getting only "MIDI out: packet assembly overflow, contact MAMEdev!" messages instead of real sound

overall: great info, Vas!! thanks a lot!