The NavVideo thing is just a switch that does exactly what he is talking about doing only by adding an input to the factory screen. If you are proficient with an oscilloscope, you could find the RGB signals and the H-Sync and V-Sync and build a switch. I think your bigger problem is going to be resolution on the nicer screen. You'll need to build a line doubler or an upscaler to scale the factory output to the higher resolution, unless you were going to use the factory screen as your carpc screen but then your stuck lowres and non touchscreen.

P