Can't access my information at the moment, but I know exactly what you are talking about. This form of "belly rot" is actually a sign of when the animal's blood goes septic from a bacterial infection inside the body (usually a rodent bite or some invasive injury is the cause) In this case, the brown around the edges of the white belly scales resembles the beginning to belly rot but is actually the end stages of the progress of the illness of the snake. They do not live long after the brown shows up.
Will post more info, if no one else does, later tonight.