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Some tube based Ham radio linear amplifiers have ~1-5KV on the plates and use ceramic rotary switches to change plate coils for band switching. Such a part could work here.
After reading Ben's post I was thinking what if that 'stepped potentiometer' had a pot as one of its resistors, and a switching arrangement such that the pot would change positions in the string? If you did something like a 6-12 position switch instead of a 3 position you could use lower value resistors and potentiometer and have less drop on each resistor and less chance of switch contact arcing.
The following probably is not mechanically practical, but it would be interesting to mount the body of the pot to the shaft of the rotary switch such that when the pot is rotated to it's stop some additional force would click the switch to the next range in the direction you're turning it...Thus keeping the control on a single knob and retaining access to its full original range.
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