Combine all the marinade ingredients in a small saucepan and heat over medium heat until very warm; do not boil. Let cool. Reserve
To butterfly the poussins, insert a heavy, sharp knife at the neck cavity and carefully cut down one side of the backbone. Cut along the other side to separate it from the body. Pull open the sides of the poussin, turn it skin side up, and press firmly on the breast with the heel of your hand to flatten it. Trim the excess skin at the neck end. Starting at the neck, gently slip your hand underneath the skin and carefully detach it from the breast meat. Slip 1 medium or 2 small tomatoes under each side of the breast skin. Fold the wings back against themselves so they lie flat.
Place the butterflied poussins in a large nonaluminum container. Pour the cooled marinade over top and turn to coat. Cover and refrigerate, turning occasionally, 2 days.
Let the poussins warm to room temperature. Heat the grill.
For the sauce, combine the reserved
Remove the poussins from the marinade, reserving the marinade, and sprinkle with salt and pepper. Grill or broil skin side away from the heat, basting often with the marinade, about 5 minutes. Turn and continue to cook until juices run clear when the thigh is pierced with a skewer and the skin is golden and slightly crispy, about 5 more minutes. Serve with the sauce and lemon wedges. Accompany with Braised Lentils with Pancetta and Aromatic Vegetables or a dish of buckwheat ravioli filled with ricotta, goat cheese, and greens, and sauced with sun-dried tomatoes, olives, basil, and cream.