Hello cobbers,
You dont say what car you have, but many cars these days have a folding rear seat, and with some models you if open the boot and drop the seat, you can actually see out through the back.
Now it will depend on which model of car, but with some the boot opens down to the bumper line, so in both my cars (Saab 93 and Renault Espace)I can see the top of the ball.
With some cars (Mainly estates) you fix a mirror on the tailgate that allow you to see the hitch. Perhaps a similar thing could be done fitting a mirror to the caravan Jockey wheel on an extension pole to give a view of the hitch from inside the car.
There's an idea for a manufacturer!