Spray 4- to 5-quart slow cooker with cooking spray. Place onions in slow cooker. In small bowl, mix dried minced garlic, cumin, salt, pepper and 1 tablespoon of the margarita mix to form paste. Pat mixture on top and sides of roast (if roast comes tied or in netting, remove before rubbing with spice mixture). Place roast on onions. Pour remaining margarita mix around roast.
Cover; cook on Low heat setting 8 to 10 hours or until meat is tender.
Remove pork; when cool enough to handle, shred with 2 forks. Return meat to slow cooker; stir in 1/4 cup cilantro.
In small bowl, mix Cilantro-Lime Mayonnaise ingredients. Slice each loaf of ciabatta bread in half horizontally. With slotted spoon, spoon pork mixture onto bottom halves of bread. Top each with 3 to 4 lettuce leaves. Spread about 1/2 cup Cilantro-Lime Mayonnaise on top halves of bread; place over lettuce. Cut into slices. Discard liquid in slow cooker.