This same problem used to occur on the plastra of Radiated Tortoises which were kept at St. Catherine's Island, GA. I have seen very remarkable results when the white areas of shell rot were gently brushed off with a wire brush (be careful) or with a very stiff bristle brush, and then painted liberally with Gentian Violet. This should be repeated avery 2-3 weeks for about 3-6 months, depending upon how well it responds.
One of my friends had a Radiated which came from St. Catherine's and looked just as bad as this Redfoot. He treated it as described above for 6 months. Not only did the problem go away, but over the next few years, the entire plastron filled back in and today looks almost 100% normal. The tortoise is a great producer of offspring for him as well, so that story had a happy ending.
One note about Gentian Violet: it is very effective on fungus, but will stain anything it touches purple, so be careful when using it. Also, it should be available in many drug stores.