>>11108942I can think of a few solutions that don't require the assumption of uniform density.
1)
Spin the body like a record with the neck at the center of the rotation. The centrifugal force is now pulling the head in the oppose direction as the lower body. The net force on the entire body is the difference between the two. From this you can figure out exactly what percentage of the person's mass is above the neck.
2) Submerge the person in water up to the neck. Figure out the volume displaced by how much water in the tank rises. Measure buoyant force with a strain gauge connected to an anchor. Repeat again with the head fully submerged. Buoyant force comes from the difference in mass between a submerged object and the mass of water it displaces.
