However, I'm pretty certain he was always aware of the possibility of signing something with one of satoshi's strongly conjecturable keys to be pretty damned solid evidence for the claim.
Block 0 key, that's the only thing that would work. You can't do that, you ain't him.
Somehow, you came into the possession of said key, you sign, you still ain't him/her/thems.
You're him/her/thems, you lost the key, you can't sign, you're still him/her/thems.
Block 0? Only?