Instead of going with an automatic "100 posts and you're in" policy, how about a more positive approach?

I suggest that new members be required to get recommended by a moderator -- any moderator -- before being allowed to post on Omni, Spam, or Coupons.

If someone has established a good record on one of the travel boards, and wants to go to the others, he can ask the moderators of his "home board" to recommend him. If one does, he is in. If none is willing, then he will have to wait a bit longer.