Preheat oven to 140°C. Place chicken, breast-side down, on a board. Use kitchen scissors to cut along either side of the backbone. Discard. Turn chicken over and press down on the breastbone to flatten. Place in a roasting pan.
Combine paprika, coriander, cumin, oregano, thyme and celery salt in a bowl. Season well with pepper. Sprinkle spice mixture evenly over the chicken. Pour in stock. Cover tightly with foil. Roast, basting occasionally with pan juices, for 1½ hours or until chicken is very tender.
Increase oven to 200°C. Remove foil and roast for a further 20 mins or until chicken is golden brown. Set aside to cool slightly. Use 2 forks to coarsely shred the chicken, discarding the bones.
Meanwhile, line a baking tray with baking paper. Place bacon in a single layer over lined tray. Brush evenly with maple syrup. Bake, turning occasionally, for 10 mins or until caramelised.
Heat a chargrill pan on high. Cook the bread for 1-2 mins each side or until lightly charred.
Place a bread slice on each serving plate. Spread with half the mayonnaise. Top with half the lettuce, tomato and chicken. Top each with another bread slice and spread with remaining mayonnaise. Top with bacon and remaining tomato, chicken, lettuce and bread. Secure with skewers.