Uh-oh, it's the dreaded "does not work" ;) What happens with your existing rule? A plain 404, with no visible change to the browser's address bar?
Are you on shared hosting? You probably said at some point, but I forget. Presumably yes, since your pattern doesn't begin with a directory slash. If yes, then a RewriteLog is out. Another quick test is to temporarily change your rewrite into a redirect, either by shoving in the [R] flag or by adding the full protocol-plus-domain to the target. You will now see the browser's address bar changing-- or not changing, as the case may be.
Can you navigate to the long icky URL by entering its address manually? You may need to comment-out any existing redirects that were intended to intercept this kind of request.