I was thinking about this too, but much stricter: Forget about the rank-requirements, and only allow posts from users who earned at least a quarter of a Merit per day (since the introduction of Merit, or since registration for newer users). If a Newbie registered 2 weeks ago, and earned 4 Merit already, he should be allowed to post there. If he's not worth it, he'll lose access in a few days as he doesn't receive more Merit.
If a Legendary hasn't managed to earn about 50 Merit since January, he won't be able to post there. I'm not sure if a quarter of a Merit per day is the right amount, but this can be changed to any value that works.