Dan, a private forum within Pprune would be the way to go, I feel. Verification of eligibility is relatively easy if people can use their real identities (with licence and employee numbers, where relevant) to register, yet may use alter-egos to post.
On topics where specialist input is called for, you could invite known experts from ATC/Engineering to view and comment on threads as required. Or, if you prefer, you could open the forum to invited people from these or other fields on the basis of their knowlegeable and valuable posting history on the main forums.
I don't think there's anything to be gained by having the forum readable by people not qualified or invited to post. I also think that such a forum would need considerable encouragement from the nominated mods to get discussions going and keep them lively - the danger being that people would drift back to the open forums if they found the cut and thrust of these to be more interesting than the limited-access variety.
A charge to enter may actually encourage participation on the grounds that if you've paid, you might as well get involved. If you haven't paid, there's less incentive to stay.
I'm in two minds as to whether it's a good idea, but I think it could be done reasonably easily, and it would at least answer the criticism that many of our debates are trivialised by non-expert input.
Scroggs