Remove and discard giblets and necks from hens. Rinse hens with cold water; pat dry. Remove skin; trim excess fat. Split hens in half lengthwise.
Combine the mustard seeds and celery seeds in a small skillet; cook over medium heat 1 minute or until toasted. Place seeds in a blender, and process until ground. Add honey and the next 7 ingredients (honey through garlic); process mixture until well-blended. Place hens and 1 cup honey mixture in a shallow dish; cover and marinate in refrigerator 8 hours, reserving remaining honey mixture.
Remove hens from dish; discard marinade. Place hens on grill rack coated with cooking spray; grill 25 minutes or until juices run clear, turning and basting frequently with the reserved honey mixture.