A standard text format for "sig codes" that can be sent between users is what multisig needs.
Can this become a BIP? Something like this can become a standard that all the wallets and other services can support.
But, ...
Would it be better to use JSON clear text, so the user can see what tx/out/etc the sig refers to? Otherwise, users will get several sigcodes mixed up and go crazy.
There is very little infrastructure right now requiring interoperability. Turning this into a BIP before we've even fully spec'd out the design space would simply slow down our ability to innovate these kinds of features. I'm not against interoperability, but I tried it with BIP 10 when I was doing offline transactions, and everyone on the mailing list was kind of like "meh, none of us are doing offline transactions, you're on your own" (I don't remember exactly what was said, but it certainly didn't go anywhere -- I tried twice).
The one exception to this is BitPay with Cosign. I actually talked to RXC from BitPay and he basically said the same thing -- we're happy to do our own things at the moment get all the kinks worked out with our own users, and then create a BIP and both/all commit to it later.