Well the idea is still in the planning stages obviously, but I found a few wide angle mini cameras that only need a little power + the output is a normal AV output. I would draw the power from the backup lights in the tailights so that the camera only comes on when backing up. There is also obviously a backup sensor signal from the ECU which can be tapped. Any display that has an AV input would work fine. That is just one idea that would work on both stock navi systems and aftermarket displays.
I want to talk to professional installers and Nissan techs before I do anything because there may be an OEM alternative that will work way better with the stock system.. If that is a dead end, I will find out what can be done aftermarket from professional shops in my area. I will keep you guys updated as this progresses.