Cook the rice in a rice cooker according to the manufacturer's instructions. Alternatively, in a medium saucepan, combine the rice with 2 cups of cold water and bring to a boil over high heat. Cover and cook over moderate heat for 10 minutes. Reduce the heat to low and cook for 10 minutes longer. Remove from the heat and let stand, covered, for 15 minutes.
Meanwhile, in a small saucepan, combine the sake-mash vinegar, rice vinegar, sugar and salt and warm over moderate heat, stirring to dissolve the sugar.
Transfer the rice to a very large bowl. Sprinkle 1/2 cup of the seasoned vinegar all over the rice: Drizzle onto a spatula while waving the spatula back and forth. Using a slicing motion with the spatula, gently separate the rice grains while mixing in the seasoning. Fan the rice while mixing it to help it dry. Wipe down any stray grains from the side of the bowl. Cover the rice with a damp towel to keep warm.