it isn't possible now. "ACLs" - access control lists, which is what you are asking for - are a significant memory hit and also are a major hit on general DB performance. I don't see the use case, particularly for generally private servers like this, as being big enough to warrant all of the code involved at each stage, along with all of the performance impacts on the database. Such a security feature would be required on a server designed to handle hundreds to thousands (to millions, like facebook and google), but not a simple server on low-powered hardware serving a mere few to dozen.
But I'm not in charge.
