Apache logs might point somewhere, ex: if you see an odd user agent that keeps accessing that link. Can probably grep it so you only see the log entries of that thread. It might have a referrer too.
I don't think this could slow down the forum much though, depends how hard they are hammering that thread.
Seems odd someone would do this though, Hmmm is one of those batteriser topics in a top 10 page somewhere, maybe it's batteriser just trying to get other threads on top to get rid of the batteriser related threads from top 10, as it might cause it to show up higher on google or something.