I vaguely recall (from building one of them for a digital design class
about 10 years ago) that just need to run a fairly-standard UART at a
strange baud rate (possibly requiring a different crystal), and then
wire in an optoisolator between the midi port and the UART.  It seems
like something you could do with minor hardware mods to a standard PC
serial card if you were so inclined.  the code to drive teh uart would
likely end up unchanged....