Position a rack in the upper third of the oven; preheat to 475 degrees F. Set a wire rack on a foil-lined baking sheet; set aside. Mix the honey, hoisin sauce, soy sauce, oyster sauce, ginger and five-spice powder in a small bowl. Cut the pork in half crosswise and prick all over with a fork; rub with 2 tablespoons of the honey-hoisin marinade, 1/4 teaspoon salt and a few grinds of pepper. Place on the wire rack.
Roast the pork until the surface looks dry, about 10 minutes. Remove 3 more tablespoons of the marinade to a separate bowl and brush all over the pork (save the remaining marinade for topping). Return to the oven and cook until a thermometer inserted into the center registers 145 degrees F, 16 to 18 more minutes. Transfer to a cutting board and let rest 5 minutes.
Meanwhile, bring a large pot of salted water to a boil. Add the bok choy and boil until crisp-tender, 30 to 45 seconds. Remove with a slotted spoon to a paper towel?lined plate. Add the noodles to the boiling water and cook as the label directs. Drain thoroughly, then toss with the sesame oil and season with salt.
Slice the pork. Divide the noodles among shallow bowls. Top with the pork and bok choy. Drizzle with the reserved honey-hoisin marinade and top with scallions.