At the moment this is just an idea for a possible alternative design for a force plate. My current design uses four load cells but I was thinking about a way to reduce the number of sensors.
The plate would have a size of 15x15cm and would be supported by a springs in each corner. The principle would consist in the combination of a sensor (Hall effect ?) put in the center of the plate to track the distance variations between the plate and the table according to the pressure applied on the plate and a two-axis accelerometer put closer to an edge to track the inclination according to the xy position where most pressure is applied. So the actual distance matters less than the distance variations, which would depend on spring's elasticity.
Thanks for the references. In between I have also found a supplier with a large choice of magnets: http://www.supermagnete.de/eng/