Trim pork of any excess fat. Scatter onion over bottom of slow cooker and place pork on top. Add broth, cover and cook on low until very tender, about 8 hours. Remove meat and let cool.
When cool enough to handle, pull meat into thin shreds, removing all fat and gristle. Skim excess fat from liquid in slow cooker.
Return pulled pork to slow cooker and stir in barbecue sauce, mustard, honey and soy sauce. Season with salt and pepper. Cook for 1 hour longer on low. If desired, serve with hamburger buns and sides of pinto beans, corn, tomatoes and coleslaw.