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op-amp input current with voltage on input during powerdown

I have an application where capacitors for a backup supply are charged to about 2.6V through a diode from 3.3V. I want to be able to monitor the capacitor voltage using a circuit that draws very little current (<10nA) from the backup supply when the main 3.3V supply goes down. One idea I had is to use an op-amp follower (powered from the main 3.3V supply) to buffer the backup supply voltage (see conceptual diagram). However if the amplifier is not powered but still has 2.6V on the input then it could damage the amplifier or at least result in excessive input current.

I can't power the follower amplifier from the backup supply since it has to drive a voltage divider (and supply current would be too high even with no load).

I don't think this approach is feasible but wanted to ask in case ADI has amplifiers suited for this type of thing or if someone has an idea for a modified approach.

Thanks - Jason