Any thoughts on a small capacitor that could be discharged to ground through a push button by the filler. Obviously it would be important to know how long it would take to discharge without the button pressed (for example if left unplugged for a while), and that it could be discharged properly in the timespan of a button push.

You could sense it's state and if it's been discharged, set an output to recharge it.

My $0.02

Aaron