Well, as for the screen, there's probably lots of ways to do it, but I just saw this a few minutes ago on eBay. The nice thing is it will display three different inputs at a time. I was thinking someone could have their side mirrors replaced with cams as well, and display all three cams at once. For your usage, you'd maybe want to stack two of these, and display all six at once. With the backup input, you can have whatever's in Input2 take up the whole display when not needed.
Dunno if that helps any, but it's a start.