To make the glaze, combine the celery and mustard seeds in a spice grinder and coarsely grind.
Combine the ground seeds in a blender with the honey, mustard oil, mustard powder, garlic, cilantro, vinegar, salt, and pepper.
Pulse on and off for about 15 seconds.
Place the hens in a large nonreactive glass dish or container.
Add half the glaze, cover and marinate in the refrigerator for at least 8 hours or overnight.
Prepare a medium fire in the grill with the coals piled on one side of the grill or with only one side of a gas grill fired up.
Place the hens skin side down on the grate opposite the heat source for indirect heat, cover the grill, and grill for 20 to 30 minute, turning frequently and brushing with the reserved glaze, until the meat is tender and cooked through and the juices run clear.