limiting new threads in a forum to users with x numper of posts in other forums

[Aneurysm]

limiting posting in one forum to users with x number of posts in other forums

I know I have seen this question answered but I can't find it. I'm looking for help with the following:

I would like to limit posting of new threads in one particular forum to members that have a minimum of 5 posts in other forums. I would still like these restricted users to be able to reply in this forum.

I have thought of using a second "registered users with x posts" group that a user would be promoted into after their 5th post on the board and then using permissions for the forum to allow that usergroup to post. I think it would be easier then that but haven't been able to find anything. Any Thoughts???