Yep, I understand you were asking about locking down categories. There's really no secure way to do that in MediaWiki. So I was offering a "next best thing" in FlaggedRevs, because sometimes requirements are flexible. (You came to us with a particular solution in mind - locking down categories - but perhaps the underlying business need might work with other solutions.)
The non-recommended, non-secure, "security" extensions are found here: http://www.mediawiki.org/wiki/Special:WhatLinksHere/Template:Page_security_e...
DanB