Combine all of the marinade ingredients in a bowl. Place the guinea hen in a large, deep bowl, pour the marinade over and inside the hen, cover the bowl, and marinate, refrigerated, overnight, turning occasionally to ensure that all parts of the bird are covered.
Preheat oven to 375 degrees. Remove the hen from the marinade. Reserve the marinade. Put the hen in a roasting pan and brush with 2 tablespoons of olive oil. Turn the hen breastside down in the pan and roast for 20 minutes. After 20 minutes, turn the hen onto its back, decrease the oven temperature to 325 degrees, and roast for 1 hour. While the hen is roasting, place the reserved marinade in a saucepan, bring to a boil, and reduce to approximately 1/4 cup. When the hen has 15 minutes left to cook, pour the reduced marinade over the hen and return it to the oven for its final 15 minutes of cooking. Remove from the oven, and let the hen rest for 10 or 15 minutes before dividing into serving portions.