What a great idea and information provided, i think if this can take place on the blockchain it will be epic and enable musicians and sound engineers and such work collectively, synergize and create amazing stuff.
Also on the multitrack forums and sites u suggested i never heard or been to any but thanks to ur info i will check it out, btw i graduate from Pointblank music school in London we do similar initiative through some musicians friends, its a small group we share multi-tracks and vsts and we use dropbox atm but def has its limitations.

Great ideas u got going and looking so forward to see how its going to manifest, Good luck and Kudos!