a five-point harness with the buckle on his back
Yeah, about that... three of those five points are pretty well useless, and the two which actually could act to restrain him in a collision are going to immediately break both his collar bones. I can say from breaking one collar bone in a three point harness (in an entirely normal, certified GA airplane), that a shoulder harness is a good idea for an upper torso restraint. As for a whole body restraint, acting on the collar bones.... a much less good idea!
This is why such airplanes are identified as "experimental". It's as much an experiment to see if the pilot can survive, as if the airplane will fly!