We're discussing this here right now (or were a page back - or to be fair it was mainly lunat who was interested in exactly what you're talking about):
http://talk.maemo.org/showthread.php?t=64921&page=4

I think at this stage, UBoot is better, because it has a headstart, but honestly, that sounds like a really good idea. IF it works. The guy who tried to test-load a kernel without flashing said his on-board flasher wouldn't cooperate. If you could make it work, it wouldn't be an advantage, but it wouldn't be worse either.