Heat grill to medium. Cut 4 sheets of heavy duty aluminum foil large enough to hold a chicken breast and veggies.
Place chicken in the center of foil and divided veggies/pineapple evenly between packets
In a small bowl, whisk together BBQ sauce, reserved pineapple juice, soy sauce and garlic. Spread about 2 Tbsp of the sauce on the chicken pieces and reserve 1/4 cup of sauce for later.
Fold up foil around chicken. Place chicken packets on the grill and grill for about 13-15 minutes, flipping over at about 7 minutes. To serve, carefully open packets and baste with remaining sauce. Garnish with green onion if desired.