I have to agree with micalbrch here....I have a large problem visualizing the the application.
The only thing that comes to mind is a solution with ceramic materials either complete, or as lining. If other (metallic) materials are to be used in addition, either sufficient cooling or insulation, or both, must be envisaged. Sizes are not mentioned here, but ceramic valves does exist for lower temperatures. This high? A needle-valve construction with a 90 degree-bend and the needle operated from the outside of the bend?