Preheat a gas grill to medium (about 400°F) on one side, or push hot coals to one side of a charcoal grill. Combine first 7 ingredients in a small bowl.
Place chicken, breast side down, on a cutting board. Using poultry shears, cut along both sides of backbone; remove backbone. (Discard backbone, or reserve for stock.) Turn chicken breast side up; open the underside of chicken like a book. Using the heel of your hand, press firmly against breastbone until it cracks. Tuck wing tips under. Loosen and lift skin from chicken with fingers; spread herb mixture under skin.
Coat grill grate with cooking spray. Place chicken on grate, skin side down, over hot side of grill. Place a cast-iron skillet or aluminum foil-wrapped brick on chicken to flatten. Grill chicken 8 minutes or until well browned. Turn chicken over, and move to unheated side of grill. Place skillet or brick on chicken. Cover, and grill 35 minutes or until a thermometer inserted in thickest part of breast registers 165°F. Remove from grill; let stand 10 minutes. Remove and discard skin; cut chicken into pieces. Serve with lemon wedges.