When I saw this as the first line of the recent patch notes I got excited..
" - Updated music tab. Added new music and fixed several playback issues "
While I would wildly guess back is that I don't think its nearly as difficult as you say, I am kinda assuming it will be left to modders to accomplish this. I'm pretty sure there are games out there that can use multiple formats of audio or combinations of soundbanks and audio files. But what do I know, I'm not a mod maker or game designer.
I just want to play custom music.
Thanks for all the other updates in the patch at least guys. I really hope GM mode continues to grow and get smoother.