(not that some of these are not already available when you know where to search, but it's not very... structured)
The "open source" format/practice exists already: just bounce your mix into separate audio files, one for each track or group, into a folder, zip it, ship.
Only, it's not (yet) much embraced on the commercial side. When you pay up to 20 boxes to get a full album, why couldn't you pay, say, 100 to get the same album but with separated tracks for your own use + instructions as who to contact & how for any other kind of uses?