AS for SID support, here is the source to the best sounding sid player for mac: http://www.fruitz-of-dojo.de/download/SIDekickv1.1src.dmg.gz
really the mixer and the filter and all of the extra code in this are not needed or wanted by me, and they distort the beauty of the original panning and mixing effects that people like martin galaway and rob hubbard worked so hard at. most .sid's play perfectly. and it is high enough quality to incorporate into AO i believe.
As for midi, there is not much reason to port to GM. However just sending a simple multichannel midi stream with velocity would be more than sufficient. Yes when output there should be an option of setting program changes at the first noteon for each channel and bpm But just having a file that results in a piano quartet would be useful. Most people wanting the midi out feature are probably musicians wanting to sample beautiful phrases like they did from records. So to me, either exporting, (&/or dreamily: having streaming midi) without any program change or extra sysex [[only note on note off, velocity and pan]] would be all i need to bring videogames into a whole new era.( i am biased because i make music mostly by recording my own midi). P.S. i will gladly remix a trans-platform song showing off the new feature, and give it to the AO project