I've been looking into making sm64 music and well your current system sucks. Especially from a standpoint of somebody who is composing and not just porting. The intermediate XML format lacks a lot of key features (velocity, pitch bends) and is a pain to edit by hand, not to mention XML just doesn't really make sense for this kind of thing. I'm thinking about making something more in line with the smw addmusic format but any input is welcome. The idea is to make it human-friendly without abstracting too much away, and to support track commands such as pitch bend, vibrato, reverb, etc. My computer is currently broken until lenovo imports a part from God knows where for me but in the mean time in going to try to design the format specifications.
I do not really understand why the question, I guess everything what is supported? (if you are talking about in inbetween format)
Anyway I plead for a direct midi to m64 conversion. Its probably the most easiest and midi itself contains basically every command ever used (except for loop). Besides that there are many editors for midi and most music people want to use are in midi format.
But I guess you want to make a inbetween format, which is a good idea to edit fast small stuff. But m64 works in layers when using more notes at once in a channel, how are you gonna display something like that without making stuff unclear? I personally do not like the way how smwc music is made