It's definitely possible to use the 200mm OR the 100mm extensions with GFIII+MCG

But to put the extensions on top of each other? Technically would work, in a combination of hard centering cams, heavy springs and pretty tightened dampers, but the hand travel to guide the grip would be so wide you'd have to move your body around to achieve max deflection.
It's one of those things, like the max speed on your car which is higher than the legal limit. You could push it, but you're on your own
