When i was running with my 4 cctv cameras and the DVR, i output the signal from the DVR to my 7" headunit.
I dont think its legal here to have footage running while driving, and it did attract a lot of eyes from other drivers, especially at night.
I too would prefer a dedicated reverse cam, witch is quite cheap these days, when reversing its the 0 - 5 feet or so behind the car that is interesting