Can it be set up that if a User tries to access threads and forums they don't have privileges for, it redirects them to the pages of how to become a Member?
Cool idea. Not sure if it can be implemented on a per-forum basis with vB but I'm certain changing the generic "you aint allowed to do that :twak:" message would be possible. It might get wiped out when the vB upgrade goes through though, I don't know if vB keeps all the strings and messages in a language-dependent file somewhere or if they are embedded in the source.
vBulletin does a pretty good job of keeping strings in a locale table, and that's a good development practice (makes it pretty simple to translate languages.) Not sure if that's difficult or not, but it almost certainly won't even be considered until the forum upgrade goes through.