Combine lemongrass, fish sauce, brown sugar, 2 tablespoons coconut oil, garlic, and black pepper in a blender; process until smooth, about 30 seconds. (Marinade will be loose and not emulsified.)
Tuck chicken wing tips behind shoulders. Run your fingers under chicken breast and thigh skin to loosen; lift skin, and spoon lemongrass mixture under skin, spreading evenly. Transfer chicken to a wire rack placed inside a rimmed baking sheet, and loosely cover with plastic wrap. Chill chicken in refrigerator at least 12 hours or up to 24 hours.
Preheat oven to 375°F. Sprinkle chicken with 1/2 teaspoon salt. Tie legs using the reserved dark green part of lemongrass or a piece of kitchen twine. Melt remaining 1 tablespoon coconut oil in a large Dutch oven over medium. Place chicken, breast side down, in Dutch oven. Cook until deeply browned in spots, about 6 minutes. Using tongs, flip chicken breast side up. Pour 1 1/2 cups coconut water around chicken, and bring to a simmer over high. Cover and transfer to preheated oven; roast until chicken is cooked through, about 40 minutes.
Meanwhile, rinse jasmine rice with cold water, gently stirring rice with your fingers, until water runs clear, about 1 minute; set aside.
Uncover Dutch oven. Stir coconut flakes, remaining 1/2 teaspoon salt, and remaining 1/3 cup coconut water into mixture around chicken. Spoon rice evenly around edge of Dutch oven, making sure rice doesn’t stick to chicken. Gently shake Dutch oven to redistribute rice. Cover and return to oven. Bake at 375°F until rice is tender and most of the liquid is absorbed, about 15 minutes.
Remove from oven, and let stand 10 minutes. Fluff rice around chicken with a fork. Garnish with additional coconut flakes, scallions, and chiles. Transfer chicken to a cutting board, and cut into pieces. Serve with rice.
