There are several cages that go up and around the back seat. As for legality, probly not ideal, but as for not dying in a roll-over and still having a back seat, may save your life.

Here are a few:
Cusco
HKS

I'm not sure which one I'm going to use yet, or more likely have one bent and made for my car specifically, as per my requirements, but I want to use the rear seat. I think with the high bars that follow the roofline, and some high density foam, they should be alright on the street, although, i would like to hear opinions on the matter.

I track at least 2-3 times a month during the season, but still need to dd the car.