I think it would fall into the definition of con traspie, but could it not danced so that at or just before, the left back step, your centre of gravity coould be behind your feet?
Yes, and I would imagine that would be the best way to lead the change of direction. That's not quite the same thing as using part weight, though, but more a matter of being deliberately off-axis (in a pendulum-type action), surely? I can't have closed my feet and not collected, and I can't have changed weight to step back, without my full weight being supported by my standing RF.