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The Australian live steam community apparently did a lot of research on using stainless steel in the model boilers several years back. They even went so far as writing up boiler code around using duplex series stainless steel. How well the practices would scale up to the full size locomotive boiler I can't comment. Typically speaking our boilers are built with far more safety margin in the plates than the full size locomotives. 3/8 inch plate scaled up would be several thick inches thick for example. However anyone who's looking to learn more can purchase the code from these people. https://ejwinter.com.au/products/ambsc-boiler-codes

This I can say, information is from qualified persons who work with boilers. The stainless used in Australia is used in the US in steam heat exchangers. Not fired, but used with steam. I’m not saying it’s safe for boilers but it’s not something strange in the industry.