This is a really good idea. I think it also might be a good idea to let all people see the thread title and maybe a short description by the moderator, but not the content, so that they know that a discussion exists and can ask to join it if they're interested. It would give people a chance to escape from the stifling moderation atmosphere that's currently settled over the site, and allow each user to self-sort into their own level of comfort. It will also let normal members get some moderation experience, and allow some criticism of moderation. For example, I recently wanted to discuss a controversial deleted thread, and linked an archived page of that thread so that another member could see what was actually deleted, and that post was itself deleted. It's kind of Orwellian that we can't even talk about content that was deleted without our posts being censored, and I can definitely see a private thread being devoted to documenting oversteps and trying to hold moderation accountable.
Idk. If the topic creator wanted me in there, wouldn't they have invited me?
It would be funny to see how personal it got though. Like imagine you're browsing threads and you see:
Topic title: ResurgetExFavilla is an asshole
Description: Anyone who is interested in bashing REF welcome, PM me for admittance. We will be discussing the various ways he is an asshole and the many levels he is an asshole on. No ResurgetExFavillas allowed.
Topic title: Castin is a vapid clown bitch
Description: If you cannot understand why anyone likes the vapid clown bitch and think she's a grating attention seeking buffoon with no apparent redeeming characteristics, this thread is for you. Castin is allowed in, but in a read-only capacity. She needs to hear the truth, but we don't want to listen to her talk.