I like the initial idea, but the more I think about it, I wonder how you'll be able to determine who is, or isn't, worthy (for lack of a better word) of being 'invited' to such a sub-forum?
I'll use myself as an example. I'm a long time member here, have thousands of posts. Despite that my posts start from confusion and disiilusionment of being a JW to the now, of being an oppenent of anything WT related, who really knows me? Who will vouch for me, my character or identity? I've been successful in my fade for one main reason...I am very protective of my real identity. I have managed 8 years here and only a handful, if that many, know my real identity.
About the only ones you could admit would be people who have been DFd/DAd and have already come out in the open. And that pretty much defeats the purpose of having it private... they're already out and in the open.
The criteria needed to ensure factualness of a member could exclude a lot of the traditional posters, posters who haven't shared personal info online as a protection.