I tried using my vintage electrometer to measure the input impedance directly. This uses a constant current source and can output around 30V. With the standard DCmV selected, I read 10Meg. No surprise. Selecting Hi-Z, the electrometer measure 22.4Gohm. However, this means it is putting 2.24V across the meter.
Using a constant voltage source at 490.03mV (within what the 87V can measure in the DCmV range) and using the electrometer to measure the current, I get about 23pA. So roughly 21.3Gohm.
Of course if you really needed higher input impedance for mV measurements, even this old electrometer is 50Tohms and has a resolution of 10uV. Then again, it doesn't fit in the palm of your hand.
If anyone else has the 87V, I am interested in hearing how this new one compares with others.