Injecting a quick hack I just made:
http://toolserver.org/~magnus/catnap.php?category=History_of_Washington
This will take all articles in "History of Washington" (max:1000 articles), get the /other/ categories these articles are in, and group them accordingly. A single article can show up under multiple headings. Headings with less than 5 articles are not shown.
We'll be right back at the category intersection problem again for large categories, but for small ones, it works quite well.
Magnus