I recently disassembled on of Bills footpads (in order to remove the sensor connection). The foot pad is spring loaded and has a little bar that sticks up into the foot. On the circuit board for the foot is a little IR transmitter (LED) and an IR receiver (light sensor). They are separated by about 1/4 of an inch. When pressure is applied to the bottom of the foot pad, the pad goes into the foot, and the little bar sticks up between the LED and the sensor blocking the light from reaching the sensor. SO, when nothing is pressing on the foot, the bar is down, and the LED shines light on the sensor, allowing electricity to flow through the sensor. When the foot pad is pressed in, the bar blocks the light, so the darkened sensor does not flow electricity. Pad out, circuit on, pleo screams because it's being picked up. Pad in, circuit off, pleo happy to be standing on something.
All that to say, the foot pad is actually a very simple mechanical movement switch. As long as whatever is on the feet allows the pad to move up and down freely, the sensor is not affected.