Another way to deal with lots of categories people don’t care about is to use groups that people can self-join.
Imagine a car site with groups for all of the manufacturers. All of the manufacturer categories allow reading only by those in the manufacturer group. The only way to see the Ford categories is by joining the Ford group.
This seems like a great solution to the program you describe. The downside of this approach is that anon users can’t see the messages. I don’t see a way to add anonymous to a group.