A URI is useful as a standard for one-way communication, but on-chain multisig requires many steps. multiple parties need to provide signatures, and one party needs to aggregate all the signatures and publish the transaction. This URI scheme would allow one to pass along a raw transaction in one direction, but once the signer has signed the transaction, what are they supposed to do with it? You might be thinking you could include a callback URL, like BIP72 does, so the signer can pass the signed transaction back to the organizer. However, BIP72 was created as a backwards-compatible extension to BIP70, which was designed for one-way communication. Since there is no way to be backwards compatible here, there is no need to limit yourself to the URI system. Instead of creating a new URI and using it for something it is not suited for, it would be better (in my opinion) to find a different method that is better suited for two-way communication.