A high-voltage optocoupler; cool.
I've posted a schematic for a hv opamp (400 v p-p) that uses two optoisolators as the output push-pull stage... it's very cheap and simple. A higher-voltage photodetector, like a glass power diode, sounds useful, too.
I worked once with a company in Southern California that had a neat gadget: it was a truncated cone of silicon with gold contacts on the base and the flattened apex. It would stand off something like 5KV until you whacked it from above with a laser, illuminating all the sides of the cone, whence it would conduct hard. I think they went out of business, though; it was pretty obscure.
John