Simple way to avoid current draw on resistor divider sense?
Just use NFETs as high-side switches under the 1M resistors. You might need an extra voltage to drive their gate, or to reduce your measurement range, but once you bias them in their triode region (which the 1M resistors make quite easy) their impedance will be negligible. 

Just make sure to use a FET with low leakage across the reverse diode (or better yet, with a separate body connection) and to drive their Vgs to zero or negative when off, to stay out of the subthreshold region. 

This is the basic idea. Assuming that each NFET is activated independently from each other.







simulate this circuit &ndash; Schematic created using CircuitLab

PFETs could also be made to work. But their biasing is not as clean as (1) you would have to use the sources themselves to provide some of your gate biases, (2) you would need level shifters to reach 100V, and (3) you increase the leakage paths due to the level shifters. And any gate drive whatsoever will put you in subthreshold and that means leakage.