Combine onion, apple cider vinegar, and barbecue sauce. Set aside.
Coat hens with oil, dry rub, salt, and pepper. Refrigerate overnight.
Cut aluminum cans in half. Fill cans with onion, vinegar, and barbecue sauce mixture. Place the cans on a hot grill, and set the hens on top of the cans. Grill for approximately 1 hour.
Brush lightly with barbecue sauce after 1 hour of cooking.