Can you add broadcast a stream via UDP feature?
If a lot of people broadcast popular files instead of simply sending them to one person then you could just pick up on the stream at any time and wait till it loops or find another person that is broadcasting the same or pick up the rest with a normal dl.
Another way would be to allow another party to join in a dl file already in progress, but I think TCP isn't designed for that.
Search could be modified to find currently available UDP broadcasts and then let you pick one.
The idea is to make more dl available from your ul bandwidth.