I'm not a programmer, but I thought up a hack that might not be too hard to implement:
Each forum visible to the public can only be posted to by its moderators.
Each of these has a parallel forum that can be posted to by the public, but is invisible to them. The post link in each visible forum actually leads to the parallel forum that is invisible to everyone except the moderators and admins. The moderators subscribe to these forums so they are notified of posts. They can then somehow transfer the approved posts from the invisible to the visible forums.
Is this what Paul is suggesting?