Not being DIR, but actually being curious about some things DIR, I hope people think twice before pushing for the second option.
I'm a member of Solo divers, an opt-in group that doesn't require mod approval, and it seems, overall, to be pretty effective. People know that they explicitly have to opt-in to have access to the area, and can be barred from it short of being given a vacation from SB overall, if it just happens to be something they are passionate about in a PITA way, choosing to be disruptive. It's known that you either "play nice", relatively, or go away.
Equally, I'm a member of the Pros group. In that case there is a pretty simple, clear, base requirement for what it takes to be a member, non-members aren't actually wanted in that area, a mod establishes that folks meet the credentials (reasonably inclusive, just that you have to meet some agencies definition of a pro, not a particular agency). For people interested in things, but not qualifying, there's the "Going Pro" subforum under Basic Scuba.
Unfortunately, DIR does seem to raise passions (even amongst your own group), so a "Going DIR" open subforum would probably not work cleanly -- it would still get trolls who couldn't be thrown out cleanly short of an overall SB vacation. Making DIR an opt-in area like Solo might provide the most access to all, yet still have a gentle hammer over people.
Just something to think about -- best of luck whatever happens.