Place the chicken breasts in a large zip-top bag or shallow dish. Add 1/4 cup reserved pineapple juice, 2 tablespoons soy sauce, 2 tablespoons brown sugar, 1/2 teaspoon ground ginger, and 1 tablespoon vegetable oil; seal and shake or stir to coat the chicken. Marinate in the refrigerator for 30 minutes.
Preheat the grill or grill pan to medium-high and oil the grates lightly to prevent sticking.
Grill the marinated chicken 4–6 minutes per side, or until cooked through and juices run clear (internal temperature 165°F / 74°C). Cooking time may vary with thickness; adjust as needed.
Transfer the chicken to a cutting board and let rest about 10 minutes, then cut into bite-sized pieces.
In a large bowl, whisk together 1/2 cup mayonnaise, 1/4 cup sour cream, 1 teaspoon lemon juice, 3/4 teaspoon curry powder, and 1/4 teaspoon ground ginger until smooth.
Fold the diced chicken, 1/2 cup halved red grapes, pineapple chunks (cut smaller if desired), and sliced green onions into the dressing until evenly coated.
Season the salad with salt and pepper to taste, then sprinkle the toasted macadamia nuts and optional toasted shredded coconut over the top.
For best flavor, refrigerate the salad at least 1 hour before serving to allow the flavors to meld.