According to the following website, http://www.faqs.org/faqs/atari-8-bit/faq/s...section-43.html, the MIDIMAX interface is fully compatible with Hybrid Arts' Midi Tracks.
This is what lead me to believe that I could fairly easily use it for my own software (i.e. it's just a simple serial port). But since MIDI runs at 31250 baud, and the Atari 800 can't go that high, there's got to be some sort of circuitry, and I don't know how it's implemented. I also can't even seem to get at the interface itself by opening the right port.
I don't know the MINIDATE interface.