Place pork in the bottom of the slow cooker. Pour the pineapple juice on top and then sprinkle the meat with all the seasonings. Cook on high for 6-8 hours or low for 8-10. Remove meat from pot and shred. Mix in the barbecue sauce.
For the Pineapple Barbecue Sauce
Mix barbecue sauce, pineapple juice, ketchup, and brown sugar together in a small sauce pan over medium-high heat. Bring to a boil, reduce heat, and let simmer for 10 minutes.
For the Coleslaw
Mix mayonnaise, vinegar, mustard, and salt together. Toss with the slaw.
For the Pineapple
Brush both sides of pineapple rings with butter and sprinkle with brown sugar. Work in batches to cook on a grill or fry pan over medium-high heat until golden and then flip and cook the other side.
For Assembling
Place a ring of pineapple on the bottom bun. Top with shredded pork, coleslaw, and the top bun.