So I want to be able to allow users to lock a Forum down to a Role, or set of roles.
I was thinking that I could do something like how UserRoles is done, creating a 1 - * - 1 relationship between User and Role, but in my instance it would be ForumRoles.
Or do you think that ContentPermissions have me covered?
Yes, ContentPermissions is the way to go. Of course you'll have to check the permissions in the entities contained by the Forum (like topics) as well.
My worry is that the Content Item Permissions stuff is not exactly the most user friendly stuff to user.
What does everyone else think?