Part of the reason the flywheel can only be so light is because the rotational forces are basically trying to rip the flywheel apart and the more weight the greater the force. This is one reason why you see more charging flywheel crack than the total loss ones. Obviously, the lighter you go the less forces you have but at the same time you still need enough beef to crank the engine and enough to weight that you don't plow into the handlepole when you let off the gas.
The metal required to turn the engine over isn't going to be massive so that really puts the weight requirement more on comfort of the rider when letting off the throttle. I believe the total loss systems were originally designed for racing so I'd bet we could shave some weight off our flywheels since we don't see the continuous high rpm runs that racers do However, if I'm wrong, the flywheel could grenade your cases too so it's one of those things...reward vs consequence.