Season pork roast on all sides with salt and pepper. Over medium-high heat in a heavy skillet, brown roast on all sides. Place onions, carrots and celery into the slow cooker; top with browned roast.
De-glaze skillet with about 1/4 cup beer, scraping up any bits stuck to pan. Pour liquid and remaining beer over roast in slow cooker. Cover; cook on low for 8 hours.
Remove roast and set it aside. Strain the liquid into a container, discarding vegetables. Place the roast back into the slow cooker, shred with 2 forks until it is completely pulled apart, discarding any chunks of fat you come across, and pour about 1 cup of the strained broth overtop; put the rest in the fridge to make soup with. Stir in the barbecue sauce. Set the Crock Pot on warm to keep it that way.
Serve buffet-style with buns or tortillas and if you like, shredded cheese, chopped tomato, lettuce, caramelized onions or creamy cole slaw.