Maybe a second pipe surrounding the hot one would cut the temp down a couple hundred degrees. Fire clay added to mortar withstands more heat, but you are pioneering on this one. Wish you well.
I am still in pursuit of something to use. I gave up for awhile, but now I have had another request.
1) Wrap the stove pipe with fiber glass insulation , with approx. 3 to 4 inch around .
2)Insert the Stove pipe with the wrapped fiber glass inside of a galvanized pipe 17 to 12 gauges .
3) Install Metal latch with metal screws on galvanized pipe . (pre drill will make your job easier )
4) Use Fired clay , Fly ash and fibers on Scratch quote and Carving quote.
Note : Using fibers on Carving quote are optional but I do recommend if your oven pipe reaches to 500 C .
I like to see your next project . Good Luck kathy