I got a bunch of assorted high-voltage white LEDs, from 6 to 23 volts.
They are clearly on at 5 uA in room light, plenty bright at 50-100 uA, blinding at 1 mA.
They are all pretty good, but the Seoul domed 23V part looks best.
The Seoul is 16 volts at 5uA, so a 2500 volt power supply with a, say,6M resistor, would peak at 400 uA, and run 5 uA at 48 volts.
The LEDs are cool, but the manufacturers are paranoid about actually revealing polarity marks, and some turn to jell-o at soldering temperatures.