The whole thing will have to fallback to TCP when some firewall (NOT router!) is stupid. But supporting SCTP has to start somewhere, and that necessarily involves pressure from users. (Witness the ECN problem.)
I don't think multiplexing over TCP is a good interim solution. If people start to implement that, SCTP will never take off.
The question is, do you want a technically-sound solution (which probably involves IPv6: no NAT there), or a hack which will ultimately delay implementing that solution?
I suppose Google is all about hacks, at least in this area. Witness Android. :-P