I don't see why you can't just make your own offsite forum and use that. That to me makes more sense, because you can choose your own clan moderators and admins.
A chat service would probably prove to be more useful, since most user-made clans here don't seem to get that many members. Something like Xat, where the creator of the room can make people mods and stuff, and they don't have to follow AG rules while talking there.
If you read the first few pages they seem to have it all figured out. How clan leaders and members will work out, but I don't think they specified a place where they can communicate, other than the obvious profiles.
I think a subforum for clans would work like the newcomers section; the general idea is for newcomers to introduce themselves, but anyone can post there. The clan section would be like that, users creating threads specifically for their clan members, but I don't think they'd isolate people from posting.