Heat gas or charcoal grill. In small bowl, mix mustard and juice concentrate; reserve 2 tablespoons. Into remaining mixture, stir garlic, rosemary, salt and pepper; set aside.
When grill is heated, rub chicken breast halves with oil. Place chicken on gas grill over medium heat or on charcoal grill over medium coals; cover grill. Cook 5 minutes.
Add pineapple to grill; brush with rosemary mixture. Turn chicken; brush with rosemary mixture. Cover grill; cook 6 to 8 minutes longer, brushing chicken occasionally with rosemary mixture and turning chicken and pineapple once, until chicken is fork-tender and juices run clear. Discard any remaining rosemary mixture.
Place chicken on serving plates. Spoon reserved 2 tablespoons mustard mixture over chicken. Halve or quarter pineapple slices; serve with chicken.