I think it is a great idea too!
I am actually very surprised that musicians are not offering U.tracks.
quote:"There could maybe be a short free UserTrack..." This is the only way to go
My only concern is, it has to be priced reasonably. I did a wish request some time ago about this. I do not remember all of the specifics, but perhaps it could work similar to kickstarter:
Offer short demo free -> get several committed folks / early birds to get project done.