Any recommendations of a supplier for a valve under these conditions? Has anyone seen of these in the field?
I have been to several vendors and have currently found only one supplier - the shaft and stuffing box are air cooled.
Most triple offset designs that I'm familiar with are only rated to 1400 or 1500 F. I should also add that this is a standard butterfly valve- no offsets - the shaft and vane are coincident with the centerline of the pipe. It is not intended that this valve fully close and seal - it's main function is only to create a backpressure.
f you are not already speaking with some of the manufacturers listed above, you should be.  If these guys have done it before, they know what won't work, too. 
I suspect you will hear the word "Inconel" mentioned as a material, if not 347 Stainless. 
Mounting the shaft horizontal will help keep the packing and the actuator out of the convected heat.  You will need a pretty long extension, and a radiation shield is also a good idea just to keep radiant heat off the actuator. 
Most heat gets to the actuator through the coupling.  Stainless steel is a poor thermal conductor.  Hence: Use a stainless coupling.  An extra couple hundred dollars for Viton seals in the actuator is also worth it.

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