1. In a medium bowl stir together the ketchup, vinegar, brown sugar, Worcestershire sauce, paprika, salt, black pepper, and chili powder. Set aside 1/2 cup of the mixture. Transfer remaining mixture to a storage container; cover and chill until needed.
2. Place pork, onion, garlic, the 1/2 cup reserved ketchup mixture, and 1/2 cup water in a large resealable plastic bag set in a shallow dish. Seal the bag and turn to coat the pork. Chill for several hours or overnight.
3. Preheat oven to 350 degrees F. Remove pork from bag and place in a 2-quart rectangular baking dish. Add remaining contents of bag to the dish with pork. Cover tightly. Bake for 3 hours or until pork shreds easily.
4. Remove pork to a large platter and use 2 forks to shred the meat. Return the pork to the baking dish and toss with cooking liquid. Serve pork on rolls topped with coleslaw and remaining ketchup mixture.