So you want to use the light circuit to trigger the pump, or will the pump run all the while the lights are on?

Wouldn't the infrared motion detector work as well. The one Metlund sells is made to work with their system. The pump would be triggered as soon as someone enters it's field of view. Even if they want to use the toilet and wash their hands in the dark! I know that in my house I don't use the light at night, but do wash my hands.