Excellent. This really opens some possibilities, at least if you’re willing to sacrifice the primary group for UX purposes (or if UX needs coincide with your existing primary groups).
Apart from hiding categories in other languages on a multi-lingual forum, I’m thinking of setting up a group for users who are overwhelmed by the UI and who might benefit from hiding various buttons.
Last year, I didn’t ask for a CSS solution in particular because it didn’t really matter to me how it was done.