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.
@gavinandresen, why not @groffer's proposal? It as simple, more powerful, and I have yet to see a clearly articulated position against it.