In a food processor or blender (or by hand), finely chop cilantro, ginger, and garlic. Add piri-piri sauce, oil, lemon juice, onion powder, salt, and pepper. Process until blended.
Flatten spatchcocked chicken skin side down. Pour ½ of marinade into a ceramic 11x7x2-inch baking dish. Place chicken skin-side down in dish, opening it like a book. Pour remaining marinade over. Cover and chill at least 4 hours and preferably overnight.
To grill, remove grate and heat ½ of the grill. When medium hot, place aluminum baking pan across unlit part of grill and put grate back on. Place chicken skin-side up over pan on unlit side of grill. Cover and cook for about 40 minutes, until thigh reads 165°F (74°C). Move to hot part of grill skin-side down and crisp skin for no more than 5 minutes, being careful not to burn it. Transfer to a platter and pour warm sauce over.