It's probably a good idea to get a product out there with this capability just to show it can be done.
Come on... do it
Could not an online wallet be configured to perform such a transaction?
That's what I was thinking. Even better, if we had a
bitcoind signfile filename.trans <address>
rpc command, the online wallet wouldn't have to hold the keys. The wallet could just allows each user needing to sign to download the transaction in plaintext, run the command, and upload the result. I don't know if they need to nest their signatures in a specific order or each sign the raw transaction, but the order could be enforced by the server if necessary.