Immediate thought would be using Latching relays trigger by the turn signal pulse.

If the other turn signal is switched, it removes ground of the previous latching relays then latches the other side giving you the other view and vice versa. Relay to remove ground on hazzard button with diode in the event of 4 ways.

Don't know if taking your eyes off the road to glance down at the monitor is a good idea though

I think I might just do an LCD rear view mirror and run either 2 cameras as previously described, or a single wide angle mounted high on the interior of the hatch glass. Would still like signal actuation.

Comments have been helpful.

im not sure if that's cool or stupid using the rear view camera triggered by turn signal.

your head should be facing the other way towards the side windows, not the center of the car.

Again, I know how to properly adjust mirrors to eliminate blind spots. This is just for fun; if I make a lane change, I'd use mirrors and a cranial rotation?

if it's just for fun, i'd embed a camera in the passenger side mirror facing down so that you can park in tight spots.

As it turns out, I can switch the BU cam on and off (or just leave on) at the appradio HU (once wired properly per the instructions) and at 170 degree viewing angle, it works as good as what I had proposed. Again, this isn't a substitute for proper mirror adjustment, but it is kind of cool.