It shouldn´t be to difficult to have a pulse generated when the reverse light turns on. The big problem is that the lilliput button cycles thru tree inputs. We had to achieve a way to generate one pulse when reverse on, and two pulses when reverse off.
It will probably be easyer to get this done by software. Use a usb cam and some type of serial controler ( Frodo have done this to disable video when handbrake is off, by using only a relay, and there is already around a small app to display a usb cam window). When reverse light is on, your app would get a usbcam window on top, when reverse off the window would go away
It would be great if possible.