good job
about hiding from certain users, it'd make your UBB slower because UBB uses cache. (it saves the HTML version of forum summary every time a post is made, so it'd make your UBB slower if the categories would be visible to only some users)
about disallowing some users to view categories when they go to ?category=X manually (if you choose to hide a category), that is possible, with just a few lines.
