Preheat an outdoor grill for medium heat, and scrape the grates clean. Spray grates with nonstick cooking spray.
In a bowl, mix together the ground beef, ground pork, bread crumbs, onion, egg, Worcestershire sauce, garlic, tarragon, sea salt, and black pepper. Divide the mixture in half, and form 2 loaves, each about 4 inches in diameter and 6 inches long. In a small bowl, mix the barbecue sauce with ketchup until well combined.
Place each meatloaf directly on the prepared grill grates, then spread each with about 3 tablespoons of barbecue sauce mixture; grill until the loaves reach an internal temperature of at least 160 degrees F (73 degrees C), about 1 hour.
