There is a suggestion forum here on the site. Repost there.
This kind of functionality would be within the capabilities of a screensaver app, developed using Roku SDK (freely available).
Someone may have created one already, do a search.
Maybe someone now will, since you have written a good idea.
I search if there is already an option but I did not find the way to do it. I hope Roku team add that feature to the Guest Mode. In addition to the Guest Mode's Custom Greeting, being able to send a short alert message to the screen will add value and utility to Guest Mode.
