I have been using SilverStripe CMS for sometime now. For a project that I was working on, I had to provide a simple messaging feature between Content Authors and Administrators.
Thus I have developed a MemberNotification module for SilverStripe. The membernotification module allows CMS Administrators to add short messages(notifications) for content authors. The content authors can login to SilverStripe CMS and then view the messages by clicking on the comment icon located on top of page.
Please refer below to screenshot, you can download the module from github.