There are a couple broad categories of injury you'd have to protect against.
Injury due to high g impact would be nearly impossible to protect against. You would have to significantly increase the stopping distance for the wearer. Given that the maximum feasible thickness of padding you could use would be an inch or 2, I cant imagine it being any more then marginally helpful.
Injury due to concentrated high force, like hitting a small branch while crashing into a tree, could conceivably be mitigated by a hard shelled harness. You'd have to be careful when designing such a harness that the inevitable added weight and restriction of movement would not make the harness cumbersome and therefore cause accidents, and therefore cause more injuries then it prevented.
I'm always looking for ways to fly safer. If you can overcome the above challenges and design a safer harness then what I have now, and sell it at a reasonable price, then I'm interested.