For the macaroni salad: Bring a large pot of salted water to a boil. Cook the macaroni in the boiling water until al dente according to the package instructions. Drain and let cool.
Toss the macaroni with the tomatoes, radishes, bell peppers, scallions and cucumbers in a large bowl. In a small bowl, whisk the mayonnaise with the vinegar, mustard, sugar and garlic salt. Add the dressing to the macaroni and vegetables and toss to coat. Season with salt and pepper. Refrigerate until serving.
For the chicken: Heat a grill to medium-high heat.
To butterfly the chicken, first remove the giblets and discard. Put the chicken on a cutting board breast-side down. Use kitchen shears to cut along each side of the backbone to remove. Trim any excess fat and discard. Flip the chicken breast-side up and firmly press each breast to break the bone and flatten.
In a small bowl, combine the chili powder, salt, garlic powder and black pepper. Season both sides of the chicken with the spice mixture. Use a pastry brush to brush the chicken with the canola oil.
Place the chicken skin-side down on the grill. Cover and let cook 10 minutes. Flip the chicken and reduce the heat to medium low. Cover and cook another 10 minutes. Flip the chicken again for 10 more minutes, then flip again and cook until the chicken reaches an internal temperature of 165 degrees F, an additional 10 minutes.
Use a pastry brush to coat each side of the chicken with the barbecue sauce and cook just to let the sauce caramelize, about 2 minutes per side. Tent with foil and let stand 5 minutes. Cut the chicken into pieces--it will be falling apart by this time! Serve with more barbecue sauce on the side and the lime wedges.