Switchable bleeder resistor for very high voltages
Using the &quot;electromechanical relay&quot;, or a mechanical switch or lever controlled with a solenoid or motor is probably the best way to do it. You could experiment gas discharge tubes.
If you want the resistance to lower with a lower current you could use a PTC resistor. With high voltage the current raises, which heats the resistor, which causes the resistance to rise. When the voltage can't keep up the current the resistor will begin to cool, which causes resitance to drop and current to rise again.