Last two ideas are ridiculous ! You can't have standees on a flight, it ain't an overcrowded city bus.
You can't seriously be talking about measuring people's height and weight to determine if they qualify for a coach seat . People pushing the limits of seat capacity damn well know they are obese.
Just force them into buying first class or extra seat or 3 for 2 , or bump them on to next available flight if nothing can be done on original flight, if they have to wait on standby for 2 days so be it, next time they will be better prepared. Spilling over into the next seat is unfair to normal sized passenger.