Step 1Toast the mustard seeds in a small skillet over medium-low heat just until they start to pop, 2 to 3 minutes. Place in a large nonreactive container with the ale, salt, maple syrup and rosemary, stirring until the salt is dissolved. Add the chickens, placing a plate over the chickens to weigh them down in the container so they stay submerged in the brine, and cover. Refrigerate the chickens overnight.
Step 2The next day, remove the chickens from the brine and brush off any mustard seeds or rosemary bits. In a small bowl, combine the paprika, salt, onion powder, garlic powder, sage and cumin to create a rub.
Step 3Lay the chickens, breast-side up, on a rimmed baking sheet or cutting board. Sprinkle about 2 tablespoons of the rub over each chicken, and begin to massage the rub all over the surface of each chicken. Flip the chickens over and sprinkle over a little more rub, massaging it into the skin. The rub will stain the skin of the chickens a rich reddish-orange as it is rubbed on. Once the chickens are rubbed, sprinkle a teaspoon or so of additional rub into the cavity of each chicken. Fit each chicken over an empty beer can, chicken rack or stand.
Step 4Prepare a grill for indirect cooking, and heat so the temperature registers around 350 degrees. Carefully place the chickens on the grill rack, away from direct heat. Cook the chickens until a thermometer inserted in the thigh reaches 165 degrees, about 11/2 hours. The chicken can also be cooked in a 350-degree oven; place a drip pan on a rack underneath the chickens to catch juices.
Step 5Cool the chickens slightly, then carefully remove the cans or racks. Cut the chickens as desired and serve immediately.