Remove pork from marinade; reserve marinade. Cover and grill pork over drip pan or over unheated side of gas grill and 4 to 5 inches from medium heat 25 to 30 minutes, brushing occasionally with marinade and turning once, until pork is slightly pink in centre. Cut pineapple slices in half. Add pineapple to grill for last 2 to 4 minutes of grilling, brushing with marinade and turning once. Discard any remaining marinade.