I agree, the hatches have enough problems. But you probably could put some stronger arms. Even though 6lbs isn't much, it is still extra weight on a already flawed design.
As far as a 4 foot stick. Imagine a teeter totter. Now, one side has a shorter board than the other. Put a heavy item on the long part. With you pushing on the short board It will be harder to get it off the ground. Just like it will be harder for the arms to push the extra weight up.
They real quest is, why would you want it on the hatch. It would go up and down with the door. So you would have to have extra speaker wire for the door just to open. Or if you ran it up and around through the hinges, that is extra cost in speaker wire and a ton of extra time for no gain. Just my opinion.
I love mine under the seat. like I said, easy to remove the speakers and it hides the amp away from the unsuspecting thief.