If you have some fabbing skills, you could make a combo body stiffener/car seat anchor in the trunk, and use some ingenuity to slip the car seat straps through the gap between the seat back and package tray.
Yes, yes you could.
I hope to be ordering these today:
Non-Retractable 4 Point Harness | SeatbeltPlanet
Upgrade your vehicle with a non-retractable 4 point harness from SeatbeltPlanet. Our new harnesses come in your choice of style and color option.
www.seatbeltplanet.com
Big thanks to ostrich for the link.