How about putting a weight platform on front of the pumps?
Fine, if there's no towbar, cockpit covers, liferafts, baggage, mud, rainwater, etc. etc. etc. to falsify the weight.
And, from my experience of dealing with manufacturers of weighing equipment, they would never certify the weights recorded by a device that was left outside and was available for public use.
And nobody could really blame them for that.