Is there an existing D7 UI solution to control the content of emails sent by the system? On most of the projects I am working on, I have set up several different emails with the HOOK_mail. On top of that you have the default user emails provided by Drupal.

What I am looking for, is an admin UI that would list existing emails and allow admins to edit their content. A bit like what you can do with the default user emails except that I want to be able to use the wysiwyg editor as well.

I started building something custom, but I wonder if there is already something out there. I guess the Message module could potentially do the trick for custom emails but I have never seen it implemented this way.
Try out the Mail Editor and see if that's what you are looking for