Preheat oven to 450ºF. Line a shallow roasting pan with nonstick foil (for easy clean-up). Or coat pan with cooking spray.
Place pork in pan and brush with 2 tablespoons of barbecue sauce. Roast until an instant read thermometer inserted in center of pork registers 160ºF for medium, about 30 minutes. Transfer pork to a cutting board; cover loosely with foil and let stand 10 minutes.
Meanwhile, whisk mayonnaise, water, vinegar and pepper together in a medium bowl until smooth. Add coleslaw mix and scallions; toss to mix and coat.
Slice pork into 20 slices. For each sandwich, place 5 slices of pork on the bottom half of each roll and drizzle with 1 teaspoon of barbecue sauce; top with 1/2 cup of coleslaw and cover with top half of roll. Yields 1 sandwich per serving.