Preheat oven to 350 degrees F.
After 20 minutes, turn pork. Remove cover and transfer pork to oven. Roast 2 hours 15 minutes, turning once halfway through.
When pork is tender, remove to a serving platter, reserving juices. Skim fat from juices and bring juices to a boil. Mix together 1/8-cup rum and arrowroot; add to juices. Boil until it begins to thicken. Season with salt and pepper.
Heat butter in skillet over medium heat. Add pineapple slices, then remaining cup of rum. Flame.