I did it! Feel free to test the requirements with your own test characters. Anyone in the Newly Registered Users group should be able to read almost all the forum (I think the moderation forum is off limits, but that is an old decision and I don't remember why it was made); post/reply only in the Introduction forum.
Here's the thing. ALL users who register with the forum are automatically added, by phpBB (i.e. there's no way to change it), to both the Registered Users and Newly Registered Users groups. There are three effective levels of permission for each possible action a member can take either Yes, No, or Never. Yes means the user can access that function fully. No means a member of that group cannot access the given function...UNLESS another group of which they are a member *does* give them access. The problem I had before was that I had selected No for all the functions we needed to restrict for NRUs. Selecting Never causes an override. So, while before, the "yes" to posting anywhere that the RUs had was overriding the "no" for NRUs, allowing even NRUs to post anywhere at all. By changing the selection to Never, the NRU settings now override.
The inherent phpBB settings are such that after meeting a small requirement (I think something like 3 posts) you are automatically removed from the NRU group.
If you find that you are suddenly unable to post, please either contact a moderator by email or PM, or leave a message in the Introductions forum.
I am hoping that doesn't happen to anyone who is actually a member who posts, but if it does, all we have to do is just remove you manually from the NRU group.
It is morally as bad not to care whether a thing is true or not, so long as it makes you feel good, as it is not to care how you got your money as long as you have got it.
-Edmund Way Teale