That’s the exact thing that can’t possibly work if you are partially supporting your weight with your feet, that’s the problem. Even standing or sitting totally still, you are constantly adjusting muscles to maintain balance, which would absolutely throw off a scale sensitive enough for the amount you lose taking a dump. And I really think that would be true even if you weren’t touching the floor, simply because scales measure pressure, which can change based on position.

It might work if you shit in the fetal position with your feet and ass on the seat, not moving, with something to support your balance… (the mental image of getting this thing to be useful is giving me the giggles)