As the Topic said, i use the Python 1.6.x Client and would have different changeable Text-Overlays, ideally running Texts if they are to wide but this isn’t a must, above the advertise Screens. This should be triggered and executed totally on the Client side. I have switches on that System, read by GPIOs, and they can e. g. trigger a Script which should blend in that extra text Information above the Ads, e. g. “No Service” on a customer desk.
There’s nothing in the Python client that would allow you to add an overlay to an existing layout without making code changes I’m afraid.
Overlay layouts are a feature that will land in the 1.8 series of Xibo which will do exactly as you want. You would need to use a Windows or Android Player to go with that 1.8 CMS though once it’s released.