Unless you have already posted an address, then a signed message is going to be of very little use to a potential buyer as any address posted after the hack should be taken with a grain of salt. It would be possible that someone could hack an account after the forum hack, post an address, post normally for a few months and then try to use a signed message from the above address to prove ownership.

I think it is pretty rare that someone would not ever post any address after being here for at least 9 months, especially considering that you must post one if you wish to ever enroll in a signature deal