Isn't this ultimately up to the congestion control algorithm that is used alongside of UDP? Aspera and UDT, for example, have tunable congestion control that can be more or less greedy.

I'm not a protocol hacker, so hopefully one will weigh in here. But I do remember some Sky Is Falling discussion over uTorrent's use of UDP for transfer, and the uTorrent line was always that they were implementing UDP transfer in a way that played nice with TCP.