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.
uTorrent's UDP congestion control algorithm (LEDBAT) goes further than playing nice with TCP. Unlike TCP, which only responds to packet loss, LEDBAT also responds to delay. This makes it yield remarkably quickly to anything else that might use the link.
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.