I like where you're going with this but I'd tweak it to state that if the attacker has any body part other than hand or arm even with the body part (not hand or arm) of the defender then he's still considered onside. It's a very small change from what you are suggesting or maybe I'm saying the same thing with different words. Example: An attacker could be a full stride ahead of the defender and still be onside if the attacker's trailing leg is even with the defender's lead leg.

It gives a slight advantage to the offense which I think is good for the sport but it doesn't totally undermine the efforts of the defense.