Trim excess fat in and around cavity of each hen; remove wing tips and reserve. Generously season cavity and exterior of each hen with salt and pepper. Reserving 2 tablespoons, rub hens with marinade and refrigerate, covered, for 6 hours.
Preheat oven to 450 degrees with rack in upper 1/3 of oven. Place onions, carrots, celery, garlic, reserved wing tips and necks, thyme, and bay leaves in a large roasting pan. Position hens on top of vegetables so they are not crowded and tail ends meet in center of pan. Roast until the juices run clear in the legs when pricked with a fork, 45 to 60 minutes. During the last 10 minutes of roasting, baste hens with reserved marinade mixture.
Remove hens from oven; let rest 15 minutes. Meanwhile, heat a grill pan over high heat. Using a pastry brush, brush cut-sides of pomegranate and treviso with olive oil; season with salt and pepper. Place cut-side down on grill pan, and cook until grill marks appear.
Transfer some of the onions and treviso to a large serving platter. Top with hens and remaining vegetables. Drizzle with about 2 tablespoons gastrique. Garnish with grilled pomegranate and treviso. Sprinkle with pomegranate seeds, if desired.