I had to do a 3 point turn on a road with a median in order to jump a friend's battery. Obviously this is risky in areas with a nearby bend in the road but we did it safely. But it made me wonder why cars can't jump other cars from the rear.

You would probably only need a red post. I'm thinking the problem with having one in the rear is running the cable that far from the battery, which would have too much resistance in the cable and the chance of a short if the insulation wears off and touches the frame. Could you not just put a fuse on the end of the cable near the battery? If a short happens or you try to start the other car with the jumper cables attached, the fuse would blow. But couldn't you have a red post in the rear to trickle charge the other car's battery? You could reduce the size of the cable and you would have less loss in the cable because the current is lower because it's made for trickle charging rather than jumping. Maybe have some kind of potentiometer that changes as a function of the voltage of the second car. This way a totally dead battery in the second car doesn't cause too much current to flow at first.