Would have to be done at the gate then, including hand luggage. But a good idea. More accurate would be some device in the struts that could display the exact weight and CG of the airplane.
Once I did a flight out of Bamako, MTOW on the loadsheet. On takeoff and during climb somthing felt very wrong. Well into the flight I went to the back and saw that the cabin was full of local males. Each one looked over 2 meters in height and 120 kilos at least. And their standard african carry on bedsheets, containing their belongings. Standard holiday weights were used, 75 kilos. As per contract, it was a charter.
As it was a long flight, we had plenty of time to study the burnoff tables. More or less 10.000 kilos above the loadsheet TOW. Using standard weights can be tricky.
Reloading the FMC with the newly calculated weight, the airplane felt normal again.
85 kilos does not sound conservative in many parts of the world but can be conservative in other parts, but using holiday weights is ludicrous. Who loses weight before going on a holiday or carries less hand luggage?