I'm guessing you'd have to add a new entry to the w3t_Posts table. It could be something like B_Hidden. The toggle would be in the editpost.php script, and a new check would be added to the post loops in showflat.tmpl and showthreaded.tmpl. If B_Hidden is 'yes' and U_Status of the viewer is not 'Administrator' or 'Moderator', then don't display the post - else display the post.
Thinking out loud as well.