Preheat the oven to 425 degrees F. Season the hens with salt and pepper and place skin-side up in a roasting pan. Roast until the skin is slightly crisp, about 25 minutes.
Meanwhile, combine the soy sauce, hoisin sauce, vinegar, ginger, 1 chile pepper and 1/4 cup water in a pot. Cut half of the scallions into large pieces, and zest and juice half of the grapefruit; add to the pot. Simmer, stirring occasionally, until thick and syrupy, about 20 minutes. Brush the hens with a few tablespoons of the sauce and continue roasting until golden, about 8 more minutes.
Peel and segment the remaining grapefruit half and place in a bowl. Thinly slice the remaining scallions and mince the remaining chile pepper; add to the bowl along with the bok choy, sesame oil, and salt and pepper to taste. Place half a hen on each plate. Stir any pan drippings into the remaining sauce and drizzle over the top. Serve with the bok choy salad and garnish with sesame seeds.