Suggestion: make it a GoogleTV / AppleTV app so users can set it up in a few clicks and transform any screen laying around into a sign.
I think BIGWORDS.PAGE would be a great companion to a web browser app on those TV devices. I haven't found any good web browser for AppleTV, but if somebody has a browser/kiosk app that let's you send a URL from your phone, that would be really great!
I built https://signagesync.app for my cafe. It can be used to create a video wall (combine multiple screens to make a bigger screen).