Noticed that the one I put the link up for has the 2nd video input specifically for the backup camera.
Assuming it automatically switches when the car is put in reverse,
it would be easy to wire a button up to make it switch.
So you could have the marquee on video 1 as the default, then when you press the button, it would switch to video 2 which could display controls.
I haven't seen any video cards with dual composite out, so it would require at least two cards.
I have too many projects started already, but it's fun to come up with ideas.