Funny enough, I've done this. However, I have no idea how it ever could make a difference. The pizza box is too stiff to touch very much of the seat. The cardboard acts as as an insulator for what parts actually do touch, and the parts of the box that touch the seat are probably the edges of the box where there isn't any pizza anyway.
You'd be much better off throwing a jacket on top of the pizza box while you drive home. The pizza probably cools off more in the second it takes you to activate the switch then the heat it would gain from the seat warmer.