If it were to be done, I would imagine it to be something a bit different. Possibly even a new "app" featuring a list of tracked users--and a Youtube-like activity feed displaying any new content posted by a user: comments, BB posts, TWiits, etc. The system would either run on the existing friend system, or on a separate following/subscribing option (like TWiiter) that could be a "Subscribe" option on user profiles and would be kept track of via accept/deny options in the app. It might even be an icon in My Zone.
That's as complicated as it'd get. It could still be just a matter of making alerts for posts in one's own Bulletin Board thread.