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.
Nice catch RE: what happens if all the if's fail-- I like the 1SUB solution, I'll check to see what FROMALTSTACK does if there is nothing on the alt stack (pretty sure that causes the script to fail), and will update the proposal to use that.
Regarding "just put OP_0" on the stack and they won't get checked" : I'm worried about an attacker trying, on purpose, to cause a node to waste CPU cycles doing unneeded signature checks.