So who gets to see the hidden material? ... but do we include theMy initial concept was "no", matching Bryce's use case.
On the other hand I generally support the notion that a member should have the right to remove their own material. I'm willing to consider modifying my stance to say that a message, while hidden, is effectively removed (to other group members it would seem as if someone deleted the topic or message), and group owners always have the right to remove content from their group. So it may be acceptable for an owner to "lock down" a message in this manner.
But it seems to me that he at least needs to receive some kind ofMods/owners can already delete member's content, and no notice is sent. So I don't think hiding would produce a larger support problem. Most likely the group owners themselves would be the first one's asked. I'm not sure if there are other use cases than Bryce's, but I'd almost expect hiding to be a less often used feature than deleting.
Now the question of unhiding a message might be a case requiring notice to the original poster, in case he/she wants to delete the message after all. That could be handled like an owner/mod editing a message (when they choose not to resend to the group).
In the case of hiding an entire topic, I still (or especially) say "no" to showing the hidden message(s) to their respective posters. That way I think just leads to confusion. Likewise, in this case I wouldn't send any notices when the topic is unhidden. From the members' point of view hiding is the same as deletion - until it gets unhidden anyway.