For at least 1 hour before smoke-cooking, soak wood chunks in enough water to cover. Drain before using.
Meanwhile, trim fat from meat. For rub, in a small bowl combine salt, marjoram, thyme, dried onion, cumin, garlic powder, mint, pepper, and allspice. Sprinkle mixture evenly over meat; rub in with your fingers.
Prepare smoker. Add drained wood chunks to smoker. Place meat on top rack of smoker. Cover and smoke at 225 degrees F to 250 degrees F for 3 to 3-1/2 hours or until tender.
Remove meat from smoker. Cover with foil; let stand for 15 minutes. Thinly slice meat. Divide meat, tomato, chopped onion, and feta cheese among flatbreads. Serve with Tzatziki.
In a medium bowl combine yogurt, cucumber, lemon peel, lemon juice, mint, garlic, and salt.