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