Anatomically, if you are interested in holding a rider in his seat should the ride fail in an inverted position, the best way to do it is to hold the rider very near his center of gravity. The simplest way to do this is to support the rider's weight at the thighs, which feature the largest bones in the body anyway. As opposed to the clavicles, which are among the *smallest* bones in the torso.
Also, please remember, Bill, just because something *hasn't* been done does not mean that it *cannot* be done. Considering the forces through a corkscrew element, I would think that...because of the applied lateral force...such an element would be *safer* with only a lap bar and a well-designed seat.
--Dave Althoff, Jr.