P.S.: Just got a message on my Commons talk page about
the
"+incategory:" search function.
* This is based on the Lucene index, right? How often is that updated?
It is updated daily. As already pointed out, it doesn't do transcluded
categories, but just looks at Category: links within raw article wikitext.
* Is there a decent interface/special page for that?
It's a pain to
enter this manually, and I doubt many people know about it
Nope, no interface. I've pretty much made it just because it was easy to
do, and doesn't really take up any significant space in the index. If
one dared to make a category intersection frontend it could possibly be
useful for testing.
However, as discussed before, making an efficient and
easily-integrable-into-WMF-type-setup backend is not exactly
straightforward.
Cheers, Robert