Preheat oven to 300 degrees. Season pork well on all sides with salt and pepper; sear in hot vegetable oil over medium-high heat, browning lightly on all sides. Add water and cover tightly; bake 2 1/2 to 3 hr until tender. Remove from oven and keep warm in juices, reserving pork drippings. Add sesame oil to a large, heavy-bottomed pot over medium heat. When hot, add ginger and garlic, cooking and stirring 2 min. Add tamari and mirin and cook 1 min, until bubbling. Add chicken stock and 1 c of pork drippings, skimmed of fat. Add mushrooms, bring to a boil, then reduce heat, simmering 10 min. Meanwhile, cook udon noodles in boiling water for 3 min, until tender. Drain and rinse under cold water. Toss lightly in a little sesame oil to keep from sticking. Shred pork. Divide noodles among 4 bowls, ladle stock over each, add a halved boiled egg, 4 oz of pork and a few tablespoons of corn. Serve onions and chilies on side to be added, if desired.