Can always do mustard or just a plain oil then add your rub patting it on the pork to make sure it sticks. As far as pulling I have a few questions for you.
Are you collecting drippings?
Are you going to wrap at any point in the cooking process?
Regardless of answers above you will want the pork butt to rest at least an hour after taking it off the pit. If you have any liquids from cooking process you would want the pork butt to cool in them before shredding. Ideally if you collect drippings you would want that meat to cool completely in the drippings (I know bark won't be crispy), but as the meat cools, it will reabsorb the liquid and make the meat tender, juicy and flavorful when you go to reheat and serve.
If you have ever had braised meats come out dry its most likely because the meat was removed from braising liquid while cooling. A tip told to me from my CIA (culinary) trained boss at my old job. Make braised meats the day before and reheat and serve if having people over for it. Hope this helps, but each person has their own way, I'm sure other will give their process too.
