Another option is to measure the compression of the suspension springs and then use the spring rates and the suspension motion ratios to calculate the corner we

Another option is to measure the compression of the suspension springs and then use the spring rates and the suspension motion ratios to calculate the corner weights. If course that would only give you the sprung weight, but you could jack up a corner and then weigh just the unsprung components with a single bathroom scale. (Subtracting any contribution from the spring, if it's still compressed at all.) Either that or remove and weigh a wheel and then estimate the rest.