After much fruitful discussion and evolution, this proposal is now a series of "Bitcoin Improvement Proposals" -- see BIP 11, 12 and 13: https://en.bitcoin.it/wiki/Bitcoin_Improvement_Proposals
Sincere thanks to everybody who contributed improvements, ideas and code.
Thanks for the feedback. I'm pushing this hard because I want much better wallet security and backup sooner rather than later.
There is no groffer proposal to pull (I think gmaxwell might take a shot at implementing the 2-of-3 case), and I don't see how to implement wallet security and backup on top of the fully-generic transactions (I'm probably just being dense).
Also, again: this isn't "either-or".