Gavin,
You have pretty much nailed my intent in both respects. This sets up a way to negotiate the address and abstract away the nasty details of finding public keys from bitcoin addresses, and provides a nice clean way for redemption abstracting away the long strings of hex.
For redemption, I think as each party signs the tx if the script returns true, it would be acceptable to just go right ahead and broadcast it, or maybe as you suggest all signatures go back to the initiating party and they do the final work.
I very much like the idea of assuming each party uses HD wallets, that certainly simplifies things greatly. I also like the use of email addresses as a negotiation medium, but I also wonder if this could be made agnostic in any BIP proposal so it could work with other communication mediums like bitmessage for example (just forward thinking anonymity a little).
I definitely think there is a need for a protocol because multisig, regardless of the application has two technically involved steps: negotiation of an address, and redemption of any subsequently encumbered funds. A protocol would enable different wallet implementations to participate in such a transaction and make wide-spread use much more likely and possible.
Drak