High Power Variable Potential Divider
You can use opto-isolators to control low-voltage DC from high-voltage DC. Wikipedia says you can use these with massive voltage differences:


  Commercially available opto-isolators withstand input-to-output voltages up to 10 kV.


This would require you to have another power rail at a lower voltage, though. Perhaps using a voltage regulator attached to the high voltage rail. This would depend on the application though.