A separate, possibly complementary feature for use cases like this that I thought of this morning would be to have a tag (or list of tags) that when added to a topic, would make the topic show up on the main topic list page.
Groups that primarily discuss things in their own, semi-private unlisted category could choose to promote things to the main page that are of more general interest by adding one if these tags to the topic.
This was be kind of be an equal but opposite feature to “muted tags”.