What if you put piezo disks -into- the sole of the shoe? Then you could read from different parts of the foot.
All conductors and semiconductors (that I know of) do change resistance (however slightly) when physically stressed. And when temperature changes (emphasis on slight in certain temperature ranges, more in others) and probably under other conditions as well.
Piezoelectric effect is not piezo-resistance.
Quartz is piezoelectric as is bone and sugar crystals (yup). A piezo lighter generates a 1500V spark to light the gas.
AFAIC, Grumpy Mike used piezo-resistance with the conductive foam under the floor tile thingus.