We don't have anything built into Ribbon yet for that. We are currently working on a major Ribbon redesign that has a more modern style and is built from the ground up with MVVM support in mind, along with other improved core concepts. That Ribbon does have an optional footer (info bar) area that can be displayed in the Ribbon. Note that that implementation is still a number of months out from being ready for beta testing though. It will be part of a Bars product we're working on.
In the meantime for the current Ribbon implementation, you could create a separate Border with rounded corners that appears under the Ribbon by several pixels to display your message. It won't be as integrated with the Ribbon yet, but you can still make it fit in nicely appearance-wise.