I remember hearing that watermelon story, too. But darn it, I can’t remember who said it, either.
Can you imagine the change in W&B with a watermelon rolling into the tailcone? How do you fix THAT? Put both feet on the yoke and push? But then what happens when you get hit in the back of the head with a watermelon!?
Nearly spit my coffee out imagining that scenario!!!
I HOPE you'd be able to roll the melon back forward without launching it into your melon... but yeah - 20 pounds way back there would definitely make for some interesting flight characteristics.