For the Brussels sprouts slaw: Heat a large skillet or saucepan over medium heat and add the vinegar. When it begins to simmer, add the Brussels sprouts. Cook until the sprouts are just soft, about 5 minutes, then remove from the heat.
In a large bowl, combine the mayonnaise, buttermilk, lemon juice, sugar, mustard, celery salt, 1/4 teaspoon salt and a dash of pepper. Whisk until blended and smooth. Add the Brussels sprouts and toss to coat. Refrigerate at least 4 hours, preferably overnight.
For the pulled pork sliders: In a medium bowl, whisk together the paprika, smoked paprika, onion powder, garlic powder, cumin, cayenne and 1 teaspoon salt. Massage the mixture into the pork, wrap it tightly in plastic wrap and refrigerate overnight.
When ready to cook, put the onions in the bottom of a 6-quart slow cooker insert. Unwrap the seasoned pork and put it fat-side up on top of the onions. Pour the barbecue sauce and chicken broth over the pork. Cover and cook 9 to 11 hours on low or 5 to 7 hours on high. The meat is done when you can easily pierce it with a fork and it slips away from the bone.
Gently transfer the cooked pork to a large baking dish and cover loosely with foil. Pour the cooking liquid from the slow cooker into a gravy separator and allow the fat to rise (or use a large spoon to skim the fat from the cooking liquid). Pour the defatted liquid into a high-sided bowl or saucepan and add the onions from the slow cooker. Using an immersion blender, puree the onions and liquid until you have a smooth, thick sauce. Season the sauce with brown sugar, vinegar, salt and pepper.
When the meat is cool enough to handle, use 2 forks or your fingers to shred the meat. To use 2 forks, hold 1 in each hand, place the tines back-to-back in the meat and pull in opposite directions, breaking the meat apart as you pull. Discard any bones, visible fat and gristle. Add sauce to your desired level of moistness and toss to combine.
Arrange the buns on a dry work surface, placing the tops and bottoms side-by-side with the insides facing up. Place about 2 ounces of pork on the bottom half of each bun, top with about 1 tablespoon of the Brussels sprouts slaw and cover with the top of the bun.