Selecting low power mosfet for battery monitor
Check reverse leakage in datasheet. I would not use a mosfet for "true zero" power devices, but a NPN (PNP) bjt, with the same base capacitor trick.

A zero base current (or under Vbe) will easily ensure some nA collector current. This kind of devices you can see in car remote controls, where a NPN is connected directly to battery, while applying (OOK) modulation to base. 

Use LtSpice to simulate.

See this
Low Drain-Source MOSFET Leakage

SIMULATED