Boilers - Stainless Steel

I don't know an dcould not say, but I suggest that it would depend significantly on the type/grade of stainless and the temperatures reached at that location in the boiler configuration.

I'd be concerned about annealing from heat cycles, softenting from the elevated temperatures and even potential sideways load from flowing gasses.

You obviously have concerns. Raise them with the person responsible for the material/solution chosen and ask them to show you how this can work (long term).

Heck, in showing you the calcs, they might realise it's incorrect. Otherwise you will have a great learning experince from a skilled professional and have some more knowledge yourself.

What is an "Acrow", and why capitalized?

Generally, SS would stand up to high temperature better than CS. Where is this brace in the combustion flow path? Is it exposed to direct flame temperature, or is it after multiple passes of flue gas?

Carbon steel grades tend to weaken above temperatures of 650°F or so, but at the "tail end" of boilers (and on through the exhaust stack), this may not be a problem.

[One cause of high stack temperatures is soot-fouled boiler tubes, which conduct heat poorly. One cause of low stack temperatures is excess air. In either case, check boiler cleanliness and tuning.]

is an Acrow.

ss dimensional stability tends to react more to heat then carbon steel. On combustion chambers we do put in ss impingment plates, but that is where for structural integreity does not play a big part

check the material is in the high carbone group with cromium content plates can with stand the heat.