Oh. Right! I made the mistake of answering what I thought he was asking (“a large number of very small forums”) rather than what his use case describes.
Yes. I’d make an “event” be either a group PM, a single topic (especially if you don’t care who can see it). Maybe create a topic, have the users forced to watch it somehow, and immediately un-list it so that it doesn’t show up to anyone who doesn’t have the URL.