I think I agree with CautiouslyOptimistic: it's too soon. I do like the idea, though.
I also agree with Angry Canada Goose about the posts too often becoming 'well, of course you're doing all you can' and striking a balance between supportive and hostile. Supportive is great: I mean, you only have to look at Team Land Crab from the old board to see what it can do.
I'm absolutely in favour of some limitations on posting rights, but at the same time I'm just not quite easy in my mind about it. Somewhere at the back of my mind is the word 'clique' and from there it's but a short step to 'mean girls' and we've been there already this month. Equally, we're all sharing nicely right now, but let's face it, we're going to get trolls sooner or later, because that's the nature of an online forum.
The only ways I can see to manage it would put a load of extra work on the mods. OK, start with automatic rules: X number of posts, membership of the board for Y period. But then I would want to add, say, no upheld mod referrals for breaking the guidelines (with a reasonable period for spent convictions, because anybody can have a bad Moment). Maybe you put your name down on a list to join once you're eligible, and a certain number of other posters in good standing have to approve? Automatic moderation on that board for Z number of posts, one warning and next time you're back to the main boards and have to go through the whole signup again? I'm trying not to go in the direction of all posts having to go through moderation because these are the only mods we've got, and we don't want to kill them with overwork.
On the grounds that there's no shame in admitting ignorance, what about talking to Captain Awkward? Many - most! - of the topics over there are on what might be called contentious topics, and she could probably tell us what to look for and what's a Seriously Just All The No in posting guidelines.