In a 4- to 6-quart slow cooker, combine the soy sauce, sugar, chili-garlic sauce, ginger, five-spice powder (if using), 1/2 teaspoon salt, and 1/4 teaspoon pepper. Add the pork and toss to coat. Cook, covered, until the pork is tender, on high for 4 to 5 hours or on low for 7 to 8 hours.
Twenty-five minutes before serving, cook the rice according to the package directions.
Meanwhile, skim off and discard any fat from the pork. Gently fold the bok choy into the pork and cook, covered, until heated through, 2 to 4 minutes. Serve with the rice and sprinkle with the scallions.