For reference,
BIP 10 is what I use in Armory to do the same thing. It was intended to be an ASCII-based way to move all this data in "blocks", either through email or text files on USB. It does work (Armory has been using it for offline transactions for 2 years), but it will be replaced soon. I'm told the payment protocol can replace this, but I still haven't had much time to dig into the payment protocol...
Of course, that dramatically increases the complexity of it, having to pass around potentially MB of supporting transactions to verify those 8-byte values. Can the payment protocol handle it?
The payment protocol has absolutely nothing to do with offline wallets or transaction signing.
Kind of... when I posted to the mailing list about making a better version of BIP 10 and asking for community support... Mike Hearn insisted that everything we need will be in the payment protocol. I took his word for it. Or I misunderstood him. If that's not the case, then I'll need to come up with another spec to replace BIP 10 that accommodates signed payment addresses, multisig, and supporting-transaction-lists.
Again, I may just be ignorant about it, because I haven't had any time to look at the payment protocol yet. And/or I misunderstood Mike.