Thanks for the replies.
Dang it. I could have sworn I was hearing tango-baker-zulu.
While I could have misheard his call sign I heard nothing about a 10-10 club either.
Judging from a Google search of the call, someone is DEFINITELY actively using KC4TBZ, as recently as this year. But it hasn't been a valid call since 1993 when the original holder changed his call.
Editing to add this link: http://dxfor.me/?_s=KC4TBZ
. It suggests someone is using it from Kentucky, 10 meters only, and DX only (or mainly). It would be interesting if someone could track them down, because this person is definitely bootlegging the call.