P-Channel Depletion Mode MOSFET for Negative Pulses
The first problem with this approach is that having the transistor ON most of the time is VERY inefficient; you are dropping 1000V across Rl most of the time. Much better to turn the transistor ON for very short pulses. It'll shrink your 1000V supply needs for a start. (This means interchanging R and FET)

You could use an N type to pull down, with some isolation (opto, or pulse transformer) to its gate drive.

If R-C is too long to meet your rise time spec, you can use a totem pole (another FET in place ofthe resistor)

EDIT : 

I just plugged a wild guess of 100pf for your lump of stainless, plus a relatively small interconnection into your stated requirements of swinging 1000V in 1 us.

Check these numbers but here's what I got.

100pf 1000V in 1 us
Q=CV = 100nC
I= Q/T = 100ma
P wasted in RL = IV = 100W.

This only considers the relatively fast slew when you turn on the FET.
Turnoff will be slower...
R = V/I = 10 kilohms
RC = 1 us so the fall time to 10% will be about 2us, maybe acceptable.