Thanks for the suggestion. That probably is indeed the closest to what I have in mind using current possibilities, assuming groups don’t work.
Do you think I can make it so that every post in that “sub forum” automatically has that tag, and the users can’t remove the tag? (I would want them to be able to remove and add other tags if they wanted.)
And then what about giving the sub-forum creator special control over the page linked to the tag, such as being able to pin posts to the top of the page, and maybe decide who can post that page?