But, in OP's case he never posted a Bitcoin address, from surfing through his post history all I did see is that he only ever posted two ETH addresses.
Any possibilities for you to make an exception? If not OP's account can be considered gone without any Bitcoin address to make a signed messages.
Creating ETH signed message are the same as creating Bitcoin Signed message, user need to be in possession of their ETH private key before signing a message, so theoretically it should be as safe as Bitcoin signed message.

While other global moderators might accept a message signed with an ETH address as proof, I'm not going to until I get an explicit acknowledgement from theymos.