As far as I know there is no standard way of doing this in Mendix. What you could try, is download the Microflow Timer Widget. This widget executes a microflow every x seconds. If you create an object in your model (e.g. MaintenanceSettings) and you allow only ONE of those object in your DB, add a boolian (ShowMessage) and a string attribute (Message), you could give the Admin of your app the capability of turning this on or off and edit the desired message. The Microflow timer will retrieve this object and show the message to your user(s) if 'true'.
However, having a Microflow timer do a retrieve on your database every x minutes is not something that I would apploud. Second downside is that every form requires that Microflow Timer widget as it only works in the form where its present. You could be creative using the microflow for you navigation buttons or somewhere else. But you need the microflow triggered in order to show the message.
Hope this helps you.
If you want to learn more about reverse AJAX, you can check out this.
Unfortunately, there is nothing in Mendix by default that can do this for you.