The A8 (I think) has side mirrors that change position when you put a car in reverse. Aside from power, what else is fed into the mirror to make it do that? Is it just a line off the gear shifter somehow? Would it be something easy or hard to try and retrofit onto an A4?

the mirror recieves a signal from the ECU when the Tranny is shifted into reverse. I doubt it could be added as a retro fit....

I am afraid he is Right, alot of work and expense for something like that. It is not only the A8 but A6 as long as the car has seat memory.
you would need the seat memory ecu, and the window ecu not to mention ( but not sure) I believe the mirror moters are different. The memory ones have position sensors