PIU are designed for extreme rear end protection. You'd still be plenty protected in the rear without the tire there, compared to a (smaller) car, unless you do as LEO might which is often stop (people) on the side of highways, or unless you put a fuel tank there which is asking for trouble, and yet people do it. You could minimize the risk with a design that can drain the tank dry and keep it empty, then only fill it up at the last gas station before your off-roading begins, then primarily draw from that tank so it is empty by the time you get back to roads.
Jerry cans seem like the best alternative.
.
