Actually, I was going to do something like this too
But, without using usergroups, 'cause otherwise I would have to make a lot of different groups
The idea I came up with: a few subfora where only the admins, super moderators and the moderators of that specific subfora can post new threads.
But then I'll need a peace of code to check wheter or not a user is moderator of that subforum.
Btw, a lot of hacks that concern moderators use if usergroup = 5, but that's a special usergroup at my board