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 forked this gist and wrote up my latest proposal here: https://gist.github.com/dba89537d352d591eb36 . Input is very welcome.