Definitely contact either of the two you mentioned (B. Amend also). You have the EWI hot tap forum as well (not a lot of activity).
http://hottap.ewi.org/resources/hot_tap/I'm struggling to see the relevance of 800 - 500C monitoring during procedure qualification. You've mentioned procedure qualification, so you've more than likely addressed burn through temps (982C id max), will complete all required testing including hardness measurements, compared the thermal cooling capacity of production situations to qualification simulation, CE's, and more than likely are applying low hydrogen processes/electrodes - H4's. Throw in pass sequence if necessary. How can you control or predict the amount of time you will be in 800 to 500C? Seems like any mechanical issues would be verified through the qualification process and measurement of such temps wouldn't be necessary. But the gentlemen you've mentioned know far, far more than I do.