We have a children's network and have turned off PMs for all levels except Mods.

Sometimes a user is exhibiting behaviors that need modification and our Mods need to be able to PM back and forth with that user to alert them of the issue and come to an agreement on what to do.

We want the private message system to allow for a member level setting to allow the sending of a private message to a user who does not have ability to send private message. 

I think this would work if: 1 Only approved member levels could create PMs. 2 Any member could reply to PMs

A difference between creation and response? Just an idea if it helps your devs. I've been puzzling on this one for a while!