Nice hack :-), but I have two questions:
you used built-in airwick time-based (time selectable by a switch) mechanism. This devices have internal squirts counter for "detecting" empty spray, when counter reach programmed limit (is notified by fast led flashing) device stops "pushing" until you press manual release button (it just reset the counter). Did you had any problem with that?
2. Did you test it only when dispenser is at the same water level in the tank (or a bit above)? I wonder if that will work when device is located below the tank with long "out" hose (about 1.5 meters) to the tank. I guess when fluid will flow slowly backwards (becouse of pump's leak) to the pump between squirts it will be unable get strict doses...