Heat 2 teaspoons oil in a large Dutch oven over medium-high heat. Sprinkle roast evenly with five-spice powder and salt. Add meat to pan; cook 5 minutes, turning to brown on all sides. Add broth and next 6 ingredients (through star anise) to pan; bring to a simmer. Cover, reduced heat, and simmer 3 1/2 hours or until meat is tender. Remove meat from pan. Cover and keep warm.
Strain cooking liquid through a sieve over a bowl; discard solids. Heat the remaining 2 teaspoons oil in pan over medium-high heat. Add mushrooms and carrot to pan; sauté 5 minutes. Add bok choy and 4 cups reserved cooking liquid to pan. Cover and cook 5 minutes or until bok choy is tender.
Cook noodles according to package directions, omitting salt and fat. Drain. Divide noodles evenly among 8 bowls. Shred the meat with 2 forks; arrange 3 ounces meat over each serving. Top each serving with 2 tablespoons vegetable mixture and 1/2 cup broth. Place 1 bok choy half on each serving; sprinkle each serving with 1 1/2 teaspoons green onions.