then i do not believe there is a listener in the core for 'before post' and after post you does you no good.  So you will have to find in the code where the post is parsed then you will have to come up with a table to handle the new privacy data and also a table for each user (or combine them).  Then you will have to add the privacy logic which im not sure will work because it is designed  to be global event in nature and not so specific, so you may have to just write your own privacy plugin.