[Farcaster]

In the absence of having this custom coded, you could achieve this using the promotions system. Setup a user group that has access to view whatever forums you want, but only has permissions to post in one forum. Then, setup a user promotion to promote the user after they have made at least one post to a user group that can post in the other forums. The only issue would be that promotions normally run only once per hour, but you could change the scheduled task to run more often.