Ok what I think you want is a system that will replace the sync signals on a video input if the original video input fails.
This can be tricky but it should be doable.
One way to do it would be to use a video switch that will select between your normal video and your replacement video source.
The replacement video source could just about be and arduino running a video overlay program. Then you are left with the task of spotting when the original video source fails. I would do this using a missing pulse detector on the line sync pulse, a 555 should be sufficient for this task. The output of this missing pulse detector then switches over the video source.
Is that the sort of thing you are after?