I agree an extra protected area would be useful for some things but if anyone wanted to, they could (via the courts) force SteveP to reveal all hidden info, so you'd still have to be careful about what you post.
Some of the areas can't be viewed when not logged in, which is fine for me.
Agree with the above comments that you could vet everyone into the 'inner sanctum' but still have a thief get through. Using a post limit isn't a silly idea as 200 posts is achievable if you post once in a while. But you have to address why you would want to do this and what it would achieve.
TBH I'd go for the 'PM me for details', baring in mind that DaveB didn't have even 100 posts so could easily be a scammer trying to build up some 'rep', in which case loads of people could have been duped.
As with everything in life, there's a certain element of risk that will always remain no matter how sensible you are.