Whisk pineapple juice, soy sauce, brown sugar, garlic, and ginger together in a bowl until brown sugar has dissolved. Pour marinade into a resealable plastic bag. Add chicken thighs and coat with the marinade. Squeeze out excess air and seal the bag. Marinate in the refrigerator, 4 hours to overnight.

Preheat an outdoor grill for medium heat and lightly oil the grate.

Reduce heat to low on one burner of the preheated grill. Place oven-safe dish over the low burner; add butter and green onions to the dish and allow to slowly melt. Remove chicken from the marinade and shake off excess. Discard the remaining marinade. Grill chicken in batches over the medium burners, turning once, until juices run clear, about 10 minutes per batch.

When chicken is cooked, place each piece into the butter and onion "bath", turning a couple of times to ensure it is well coated. Continue to grill remaining chicken, adding each piece to the butter and onion "bath", pushing already-coated pieces to the side until all pieces have been coated.