I was just browsing wordpress/bbpress plugins and found this. Maybe it would be worth integrating into the Cocos2d-iPhone forum? I looked for another post about this but didn't find one. Sorry if I am reviving and old topic.
Private Messaging for BBPress
http://faq.rayd.org/bbpress_private_message/
Private Messaging Extended
http://bbpress.org/forums/topic/private-messaging-extended
-Sunsu