I'd like to introduce the official discourse-staff-notes plugin. The idea here is that sometimes staff members want to share notes about a particular user to help with moderation or support duties.
You can add a Staff Note to any user by using the admin menu on one of their posts:
The interface is displayed in a modal (which is also accessible from the user's profile page). You can add as many notes as you need to, complete with markdown support:
If a user has a staff note, whenever they post you'll see a little notes icon. Clicking it will bring up the notes:
That's pretty much all there is to it! I'd love to hear feedback from people who install and try it out. I should note that it currently requires the tests-passed branch of discourse to work, although within a week or so it should work with the latest beta.