As a quick check - a weighbridge in the aerobridge to the aircraft. All it would need to do is calculate total weight (therefore individuals wouldn't get their nose out of joint). Ensure gate staff don't pass through the weighbridge if they are not boarding the flight (go around through a staff only taped off area). Flight/cabin crew go once only through the gate. They can weigh 62.5t trucks on the highway to a reasonable degree of accuracy as they pass over the weighbridges here in Australia at 60kph - surely people can be measured to within a kilo as they walk through. Do it at the entrance to the aerobridge so the process of scanning boarding passes spreads out the passenger flow.