Most of that lateral movement of the foot comes from tibial rotation during the pedal stroke - so the foot would move in the same direction as the lower leg and would not cause rotational friction in the ankle brace. There may be some other ankle related lateral motion due to foot motion in the shoe (i.e. pronation/ supination) but the rigid sole and tight fit try to control this as best as possible. I’m still not saying that an ankle cuff might not have drawbacks, but I think it deserves to be prototyped and tested somewhere out there.
P.S. Some of the cooler (read: more expensive) production shoes out there are made by Simmons Racing (see some pics at the link below)
http://www.simmons-racing.com/Cycling_shoes.html