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.
I've implemented initial support for these in the hopes of including relaying/block inclusion in the 0.5 release:
bitcoin/bitcoin#541
Supporting these doesn't cut off support for the more general transactions in the future.