Re: [ANN][XCP] Counterparty Protocol, Client and Coin (built on Bitcoin) - Official
Please don't get me wrong, I do not say that we don't contribute to a higher utxo set and a general statement about the usage of multisig transactions is more difficult to make, because I have no idea how to evaluate the other 99.999%. My above used criteria were: "has an standard output to Exodus address and multisig output" and "has an multisig output with XCP magic bytes". If you have any idea, go ahead. 
Even with multisig neither Counterparty or Mastecoin need to be contributing to the UTXO set: just have one of the pubkeys be valid, with a secret key known to the user, and have their wallet software spend that txout the next time it needs to spend some money.
You're transactions will be cheaper overall too. Again, this is another reason why you want to do an arbitrary encoding strategy. I'm rather solidly booked for the next two weeks, but we can talk about implementing that later as an upgrade.