Make the rub: add the mustard, paprika, salt, peppers, garlic powder, onion powder, and cayenne in a bowl; stir to mix.
Sprinkle rub all over the pork, patting it onto the meat with your fingertips; let the pork cure at room temperature while you make the mop sauce.
Mop sauce: combine the vinegar, mustard, salt, and pepper in a large mixing bowl; add in 1/2 cup water and whisk until salt dissolves.
Set up grill for indirect grilling and preheat to medium-low—for a gas grill, place all the wood chips in the smoker box and run the grill on high until you see smoke, then decrease heat to medium low.
When ready to cook, place the pork, skin side up, in the center of the hot grate, over the drip pan and away from the heat, and cover the grill.
Cook the pork until darkly browned on the outside and very tender inside, 4-6 hours.
Test for doneness with an instant read thermometer; the internal temperature should be about 195°; if the pork starts to brown too much, cover it loosely with foil.
Every hour, for the first 4 hours, swab the pork with some of the mop sauce, using a barbecue mop or basting brush.
Make the mustard barbecue sauce: melt the butter in a saucepan over medium heat; add in onion and garlic; cook until soft but not brown, about 3 minutes.
Stir in the mustard, brown sugar, vinegar, and hot sauce; add 1/2 cup of water; let sauce simmer, uncovered, until thick and richly flavored, 6-10 minutes.
Taste for seasoning, adding more hot sauce as needed and seasoning with salt and pepper to taste.
Let the sauce cool to room temperature before serving.
When the pork is done, transfer the cooked pork to a cutting board, cover it loosely with foil, and let it rest for 20 minutes.
You could pull the pork or chop the pork, but Steven likes to thinly slice it across the grain.
Place the pork slices in a foil pan if you are not quite ready to serve, cover the pan with foil and place it on a warm, not hot, grill or in an oven turned on low.
To serve: brush the buns with melted butter and lightly toast them on the grill; load each bun with sliced pork and slather with mustard sauce.
Top the pork and sauce with pickles and serve at once.