As far as I know all clients allow signed messages, except for maybe a couple of online services. Not even sure of that, since I know you could do it with Empty Gox and that you can with Blockchain.info. I think I can do it from Coinbase as well, though I haven't checked since I only use them as an exchange.

At any rate, if you can prove your address to Wizkid's satisfaction, I'm sure he can arrange something. I'd contact him.