Having an invite only forum would be fine, but those posts shouldn't be isolated from the public, instead it should be something like a +v channel... only those invited can reply and post. The rest just watch and listen.
That's exactly what's missing from both Foundation and this forum's "premium" hidden section.
You don't need to hide things, this will only feed conspiracy paranoia. Just make it read-only for non-members.
And if people can't see what's inside, it's hard to know whether it would be worth paying for.
Overall, I don't think the problem is how to do it, it's just the lack of will (or organization).
I've been off bitcoin for 18 months and came back to the same sorry state of affairs as it was back then
