It seems like you trying to sign a transaction from another application. Armory does not do that whether it's multi-sig or not.
In the process of trying to sign this transaction you have stumbled upon Armory's message signing functionality. All that does is sign a message so that you can assert that it came from you since you signed that message with one of your private keys. Rather than signing the Counterwallet transaction, you are just signing a message that is the serialized transaction.
According to google Armory has been ported to the Counterparty platform. I cannot speak to how that works. You'd need to talk to people familiar with that project.
I don't know the tech details of any of this, but I can say that I have successfully signed a transaction on an offline Armory installation, where the transaction was created in Counterwallet and later broadcast by Counterwallet. I am not using any sort of Counterparty port of Armory (if that even exists); only plain vanilla from the Armory website. Whether this process is reached through Counterwallet doing some magic to a signed message which carries a serialized transaction, as you refer to, I wouldn't know.
Oh, and when signing a Counterparty transaction (which is, in essence, as you know, just a bitcoin transaction) I do not choose message signing. I do indeed choose the "Offline Transactions" button, then "Sign Offline Transaction". You can see the complete process of setting this up and doing a (non multi-sig) transaction from a user perspective in the link in my previous post.
