Pat pork dry. Make shallow incisions all over one side of the meat, about 1/8-inch apart. Spread garlic over this surface of meat, pressing into cuts. Repeat with ginger and cardamom seeds or powder. Follow with salt and pepper and bay leaves. Add pineapple juice to casserole dish just large enough to hold meat. Add pork, seasoned side up. Cover and reduce heat to 1/4 power and cook gently for 20 minutes.
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.
To serve, surround pork with pineapple and coat with sauce.