Given how much money you've raised it'd be quite easy to pay someone to add support for OP_RETURN to those clients, among other things.
Hey Peter! Thanks for checking out the project! (Peter and I met at the conference in San Jose, and had some interesting discussions there, and he is a tireless advocate for keeping bitcoin awesome, as you can see from his sig)
I completely agree. We seem to have a rather formidable war-chest

Being a good block-chain citizen is a matter of survival for MasterCoin. If we don't, we'll face competition from a clone that says "We're like MasterCoin, but with less impact on the blockchain"