"Come to think of it, there only one short step to weighing luggage, handbaggage and person, all at the check-in counter"
I've long held the belief that charging should be based on total weight being carried. But, as a slim passenger with only hand baggage, I would wouldn't I! In fact, dont limit this to just air travel....extend it to all vehicles. It must take more energy (= fuel burn + emmissions) to transport a 120Kg person than a 60Kg one.
It would take a brave business person to try and get that idea through though :-) Mind you, there's one carrier I can think of that would probably give it a go.....