I like the idea of defining "relationships" and having that be defined somewhere. However, where would it be stored? I really think that however it is done, it must be coded into the wallet: the user should never be in the position that they recover this wallet and think it's a single-sig wallet. I want wallets to be dedicated to multi-sig or not. If a user doesn't want multi-sig, create a regular wallet. (easy for me to say, Armory is a multi-wallet app)
In my view, the wallet IS the relationship, or at least a file representing one party's participation in a relationship. Multiple relationships means multiple files, one file per party per relationship. The data defining the relationship would be in the root structure of the wallet file, along with the party's own private keys and the other parties' public keys. This would be consistent with the view that a regular wallet file was simply a "relationship" file where number of parties is 1 and that party has the one and only private key.
The proposal I speak of would preserve this ability. Although I proposed that party A issue some record proposing a relationship and party B issued a record accepting the proposal, nothing stops A and B from being a single program running in a trusted environments and producing both records and printing them to separate pieces of paper.
Given that offline computers are something that come easy for you and me, and the popularity of smartphones among everybody else, I envision the most popular use case scenario being where party A is a computer, B is the same person's smartphone. In this case, the "relationship record" goes from A to B in the form of a camera scanning an on-screen QR code, and where the returned record travels from the smartphone back to the computer via E-mail, or removable flash media, or the user just bearing it and hand-typing the response record off their phone's display onto their computer via keyboard. Private key material for A is backed up by printing it, and private key material for B is backed up by the user writing down a wordlist-encoded record they see on their phone's screen.
Once such a relationship is set up, users would initiate transactions on their computer (A), and their on-computer client would show QR code (or a series of them if the transaction was large) containing the proposed transaction and the party A signature. The smartphone would display the transaction details on screen, get the user's consent, provide the party B signature, and send it all directly to the bitcoin network itself.