Turns out, best I can tell, that published standards for high temperature IR measurements are not available. I'll be happy to be corrected anytime. Possibly some development in the works internationally. Meanwhile, I am adapting low T calibration procedures to high T situations. My challenge is that some high T equipment uses a slightly different twist on the IR flux to T conversion. Superior, but not subject to identical sources of uncertainty.
So I leave this thread outstanding: Who has experience with calibrating high temperature IR thermometers? Especially for in-plant use? Who would like to discuss the matter?