Measuring voltage across series capacitors... Impossible?
Something like the TI LMP7721 Opamp is probably what you're looking for:

In the femto-Ampere range, we can be pretty certain that the measurement current-induced error is &quot;rather low&quot;; let's say a X7R ceramic cap has some 10 GΩ resistance. At a 1 V voltage across one of these, we'd have a 100 pA leakage. That's a solid 2 to 3 orders of magnitude above the input bias current of the opamp.
Bonus: having such an opamp allows to have a &quot;voltage following&quot; shielding around the nets of interest – further reducing leakage due to potential gradient in e.g. the PCB substrate :)