High voltage comparator
I have had similar requirements for HV systems used in Photo-multipliers where the typical load is nano amps, so the supply is very small. It is not accurate to say you can not load it, because any system, resistors, comparators etc will produce a load.

Below are two options based on your needs:


[~10M-100M Ohm Load] - Use a very large resistive divider to not load your source, or since you know the exact resistive load, account for the losses in your control system etc. Then buffer the output with a JFET style op-amp or an emitter follower (Both would have typically > \$10^{12}\$ Ohm input impedance). I would use something like 10M and 1M resistors for dividers, and potentially a capacitor on the lower half to reduce noise.
[~ >\$10^{12}\$ Ohm Load] - Provide a 2nd rough high voltage supply to use for buffering (Does not have to be regulated, just higher then what your signal is). Then use a high voltage FET and create an "Emitter Follower" (Or in FET terms, Source Follower). This would provide a lower impedance copy of your signal, that could then be divided with resistors in the 100's of kilo range in the event you need a stronger signal with lower noise.


Also, just an FYI, Even high end measurement systems like 1000 USD DMMs and 10,000 USD scopes often have input impedance in the 1M ohm range. Not that you can not get a special system with higher, but if your comparing what your making to something commercially available, they do not have insane front ends due to load vs noise tradeoffs.

Also Note: the above solutions could have issues if you really are trying to measure down to 0.5V. Buffers, Emitter followers etc. might need a supply that goes below your measurement ground for this to work... I would say 10V minimum you should be safe with a 10:1 divider and a common gound.