This would be good but it may have very low priority. Till then, if you know the address, you can input it in
BLOCKTRAIL Block Explorer and go to 'Mentions'. You will find posts having that address from Bitcointalk, r/bitcoin and maybe other.
~~MZ~~
My proposal is to use it the other way around. Someone could input their username without knowing any address associated with the account and find all the BTC addresses they ever posted. If you know their address then you can use a forum search as is enabled now to find potential alts.