The 400 SS is probably martensitic to give it the higher hardness and levels to match the performance of cast iron. Something like a 440 or a a 410 might work. They would be a lot more expensive, though. 440F rotors would not make sense for me. I drive so much that rust just isn’t an issue. If someone had a weekend or seasonal vehicle that they stor outside, reduced rust rotors might make sense at a significantly higher cost. Would such a limited market be worth the cost of setting up the business? Or you could just knock the rust off at the first few stop signs.