If one looks into the Tur Beis Yosef (and Shulchan Aruch) in Orach Chaim Siman 326 you will find the halachos of bathing on Shabbos. The Beis Yosef there points to the gemorah where it outlines the list of gezeras that were made and when in the process of getting to the halacha now a days of not bathing on Shabbos. One of those gezeras is as well to not be "m'ziah" (to go into a hot bathhouse in order to sweat.) However we know that one may wash their face, hands and feet on Shabbos with water that was cooked from before Shabbos. If someone would theoretically have a pot of hot water that was made from before Shabbos may it be used to "steam" there face (to creating the same affect as one would in a case of being 'm'ziah'.) Seemingly if it would be muter to wash ones face with water heated from before Shabbos then this would also be muter. Does anyone speak about this?
(Setting aside any other issues that might come up in Hilchos Shabbos.)