>... How do you create a large lateral load on the heelpiece and not on the toepiece? ...
This would occur in any fall that is applying a "sideways" force to the knee without a "twisting" movement of the pelvis relative to the foot. One example that comes immediately to mind is a skier-skier collision in which something hits the knee or lower leg from the side while that ski is weighted (ie, like the football situation).
Another, less spectacular example would be a slow speed fall in which the person essentially collapses straight downwards and sits on the inside of their boot with their leg folded at a sharp angle under them.
Fortunately, in most falls, particularly higher speed tumbles, the application of forces and torques to the body is never as static and clearly defined as in the above examples, so in all probability a "sideways toe" or "heel up" force will eventually occur (semi-randomly), and the skier will eject from a conventional binding. Hopefully, this happens before serious damage is done.
Personally, I have a couple of pair of skis with Look bindings on them, and I do worry a bit about the lack of sideways heel release, but to me, its mostly a theoretical concern, and hasn't prompted me to replace them. On a historic note, sideways release at the heel is nothing new. The old Moog bindings from the late '70's and early '80's had toe-sideways, toe-up, heel-sideways, and heel-up (and all combinations thereof) release modes.
>... do we see this increased rate injury among users of Look/Rossi bindings? (which have arms that prevent lateral movement of the boot heel in the binding) ...
Good luck at getting such data into the light of day (if it even exists). Imagine the legal reaction of Look/Rossi if Ettlinger, NSP, or anyone with access to such data ever published it.
The issue of differential injury rates across different bindings is further confused since the large elasticity of the Look/Rossi bindings allows those who adjust their own bindings to use lower DIN settings and not pre-release. For such users, this means that even if a relatively small fraction of a load that is mostly sideways at the heel winds up as a sideways force on the toe piece, you will still come out.