mudgel
This is copied from the second of Johns links above.
"...... MIDI tracks, regions, and even tempo maps are not provided for in OMF/AAF sequences."
In principle you could store midi info as metadata - nothing in AAF format that inhibit that, as far as I can tell.
So if anybody start doing it - it might spread.
The way to do it are many - so those that start doing it one way, may get followers.
You can move midi data in music xml - so why not put such a file inside AAF, and also have start position if not already.
Or you extend current OMF export with also including midi on selected tracks in xml format beside as separate file or also start a new tag inside for it.
Midi is so dominant today, one could say, so why is it ignored?