But of course motorbikes have always had adjustable rear spring seats so they can be lifted up with a c-spanner, to adjust for the load of a pillion or an odd weight rider. Not sure if that pillion fits the criteria though.
Perhaps cars for towing might benefit from a similar system. I know some have air suspension systems. I have used shocks with air bags on them previously on an old Holden to good effect, and you adjusted the lift easily with tire valves in the boot.
