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.
Updated proposal; I think we would regret prior proposals, because ECDSA signature verification is NOT cheap, and prior proposals would have resulted in potentially lots of extra unnecessary signature verifications.
I'm taking the approach of redeemer puts exactly the right number of signatures in ScriptSig, along with a number that indicates of which case is being satisfied.
Improvements and suggestions are, as always, most welcome.