Place chicken in slow cooker. Pour reserved pineapple juice, ketchup, brown sugar, soy sauce, Worcestershire sauce, garlic, ginger, and pepper over chicken. Stir to coat.
Cover and cook on low for 6 hours or high for 3 to 4 hours until chicken reaches 165°F and shreds easily.
Remove chicken and shred with two forks. Return to slow cooker.
In last 30 minutes, stir in cornstarch mixture and pineapple chunks. Cook uncovered until sauce thickens.
Garnish with fresh green onions and serve over rice or on buns with coleslaw.