In a small bowl, combine the cornstarch, grapefruit juice concentrate, water, soy sauce, honey and ginger; set aside. In a skillet or wok, stir-fry zucchini and red pepper in oil over medium-high heat until crisp-tender, 3-4 minutes. Remove and keep warm. Add half of the pork; stir-fry until no longer pink, about 4 minutes. Remove and keep warm. Repeat with remaining pork.
Add sauce to skillet; bring to a boil. Cook and stir until thickened, about 2 minutes. Return pork and vegetables to pan; stir until coated. Gently stir in grapefruit. Sprinkle with sesame seeds. Serve over rice if desired.