Restrict sending private messages only to Admin and others to reply

The PM system works fine as it is But I would like to add the option to restrict it to "Only Super-Admin can send, Any user can reply to PM from Super-Admin". So that means hiding the PM icon (seen in the profile block, and at the top of some pages) to everyone except the Super Admin.

BUT making it visible at the top - where it normally shows {ENVELOPE ICON}(0) - to the user who has an unread message from the Super-Admin. Basically I'd like to have a private PM system that the Super-Admin can use to exchange messages with regualr users - but that system should be invisible to everyone else who is not being contacted.

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Background: our site relies on users sharing information, and having a PM system for them to use amongst themselves short-circuits the free flow of public information. In general we want all exchange of information to happen only on the public forums.

BUT, we also find that often a user's email address has changed or expired, and they have not notified us of the change. So we get email bounce-backs from the forum, when they have subscribed to topics or sections. As there is no way for us to contact them ( their email of record is not working any more!) it would be EXTREMELEY USEFUL for us to be able to leave a message (advising them that we need to update their email in our system!) for them on the forum, that they can see the next time they log in.  We used to use the udde messaging system for this, before it was retired...