You're supposed to sign the message with a bitcoin address, not a GPG key, though if you claimed ownership of the GPG key in a public post on your main account it should still work (I'm not sure if signatures count). Also, you need a link to the post from your main account with the address or GPG key in it; otherwise there's no way to verify the key is really yours.
For more information, see this thread (though I suspect you already have):
https://bitcointalk.org/index.php?topic=497545.0Actually you can find the following sentence in that thread.
"Typically, the only acceptable method of proving ownership is by signing a message (including current date and desired new email address) using a Bitcoin address or
PGP key associated with the account."
Whoops, missed that; it does make sense, since GPG is well-known security software (which is why I mentioned that it would likely be fine), but I forgot that it was actually in the post.
Also, upon re-reading the post, I noticed that it does mention signatures (emphasis mine):
A Bitcoin address or PGP key is associated with the account only if the account posted the key/address, sent it in a PM, or if it is still listed in the account's profile.
Since his Bitcoin-OTC link is still in his main account's signature (profile
here, and it's listed under the username of the account he used to post here), the request should be fine as long as the GPG signature verifies (which I'm still figuring out how to check, I haven't worked with GPG much before). Usually I only see these posts from users who haven't bothered to look for the format, so this one is rather refreshing.
To the OP: Have you tried contacting theymos again times? He's said that he tends to drop requests if he can't get to them within a few days, and with the amount of PMs he probably gets I wouldn't be surprised if that happened a few times.