A quick check of one of my small Van de Graff generators gave a reading of only 18,000 volts, suggesting that the 10 giga Ohm impedance of the probe is still drawing more current (1.8 mA in this case) than the generator can deliver.
I'll try it out on my Tesla coil soon.
Tried it with the Tesla coil. Whenever the coil was running, the multimeter display blanked out. I'll probably need an oscilloscope and a Faraday cage to make this work. Time to cruise E-Bay.