Roast the red bell peppers on a hot grill or over an open flame, turning, until charred all over. Transfer the peppers to a bowl to cool. Peel, stem, seed and coarsely chop them. Transfer the peppers to a food processor and add the jalapeños, garlic, vinegar, cumin, salt and pepper and puree. With the machine on, slowly pour in the olive oil.
With a small knife, make 1/2-inch-deep slits all over the chicken halves. Transfer to a large, resealable plastic bag and add the marinade. Refrigerate the chicken overnight. Bring to room temperature before grilling.
Light a grill. Wipe off most of the marinade from the chicken. Oil the grill grate and put the chicken on it, skin side down. Cover and cook over moderately high heat, until lightly charred, about 7 minutes. Turn the chicken and cook for about 7 minutes, until charred on the other side. Turn off the heat in half of the grill or rake the coals to one side, away from the chicken. Cover and grill, turning the chicken halfway through, until an instant-read thermometer inserted in the inner thigh registers 160°, about 1 hour.
Transfer the chicken to a work surface to rest for 10 minutes.