Rub pork with salt, black pepper, and garlic powder.
Put onion in a 6-quart slow cooker; top with pork. Pour broth around it.
Cover; cook on low 8 hours until pork separates readily under a fork.
Whisk mustard, vinegar, brown sugar, tomato paste, Worcestershire, and cayenne in a small saucepan.
Simmer sauce over medium-low heat 5 minutes, stirring until glossy. Reserve 1/2 cup for serving.
Transfer pork to a rimmed pan; discard excess fat. Shred with two forks.
Skim cooker liquid. Return pork with 1/2 cup liquid and remaining mustard sauce; cook on high 10 minutes before serving.