If you think about it, in principle, the best you could do is make a device that 1) controls the acceleration of the head assuming it will reach a certain displ

If you think about it, in principle, the best you could do is make a device that 1) controls the acceleration of the head assuming it will reach a certain displacement e.g. d=10cm with final velocity v=0 ) transfers the forces from the skull to elsewhere. So the absolute best protection you could get would be some kind of cage around the head not linked to the head at all -- maybe you couple this cage directly to a vest akin the HANS device you mentioned, and actuate a separate mechanism smoothing the head movement, with the largest radius possible around the head to provide the least maximum acceleration ( a_maxmin = initial velocity^2 / 2 * d ). Normal head movement could be completely unhindered. Now since this is a sport this could devolve into a bunch of people just bashing pieces of plastic into each other, which may or may not be desirable over the current situation; how much such a device would cost (not accessible to amateurs); and as others mentioned it's also possible their velocities will increase to compensate, maybe not changing the number of injuries at all.