I am not sure on this at all... but wouldn't it just take a little bit of modifying the "subscribed forums" part.

Where you can subscribe to different forums... and then those can the only ones seen if you select that..

like make a yes or no radio button function to allow each user to just show the forums they are subscribed to, or to show all..

dunno like I said, and this may be more work than it's worth.. or there is probably alot easier ways.