I think Linda was refering to me
If I had the time I would do this. But what I'll do is run over quickly what you should do to create this MOD (so that a less busy mod-author can do it).
1) Create new table for messages. With report writer's userid (Mod, Admin's id), user_id (whom the message is for) and the message. (you can extra fields if you want it to be for all users or a group).
2) Use some of the PM code (regarding the popup) for displaying the message.
3) Add a conditional to page_header.php (if user has a 'report' waiting for them, then display the popup with a message).
4) Add a section within MCP for sending out messages.
I wish I had the time so that I could do this. Maybe if no-one picks up on this, then I will do it at some point.