What for? Can you enlighten me that why do anyone needs you to sign your BTC address for them.
I will sign YOUR message 2011 BTC key. Thus, someone can "prove" that he is in the subject since 2011 - it's obvious!
I don't understand why would anyone want that ? And why would anyone pay 1 ZEC (50$ at current price) for that ? The purpose of signing a message from address is proving ownership of the address. This is the first time I see that someone would actually be interested to prove for how long you are "in the subject"