Very good question, I think the first step must be the creating of a norm 
Yes, I really think so, no jokeā¦did you try to squeeze a common body weight scale with a similar grip like that above?
I did and reached from 5- 25 kgs, depending on different ways to grab the scale.
Do you think it would make sense, preconditioned the axle is 100 % stiff mounted, to apply a defined weight (say 10 kgs iron attached to a hook) at the rimās edge?
Measuring of the wheel bending via clockwork would be easy then.
Iāve all kinds of Strida wheels (except 14 inch) handy and a comparison should be no problem,
but for a minimum standardization of the measuring method Iād appreciate your advice 
Btw, I canāt resist to mention that ābombproofā 16 inch Strida wheels were not builtā¦by nowā¦