Put beans in a large pot with enough water to cover by about 2 inches. Bring to a simmer over medium-high heat. Reduce heat to a medium and simmer uncovered for 15 minutes. Drain beans, reserving cooking liquid, then transfer beans to a 2-quart bean pot or Dutch oven.
Preheat oven to 250 degrees. Place salt pork in a small pot, add water to cover, then bring to a boil over high heat. Drain pork and add to beans. In a small bowl, dissolve mustard in 1 teaspoon warm water. Add dissolved mustard, molasses, and rum to beans. Season with salt and pepper, and mix gently but thoroughly.
Heat reserved cooking liquid in a medium sauce-pan over medium heat. Pour enough of the liquid (about 3 cups) into bean pot so that beans are moist but not floating. Reserve remaining cooking liquid. Cover pot and bake, checking occasionally to ensure that beans are not drying out, adding reserved cooking liquid as needed. Cook until beans are soft, about 5 hours.
Remove cover, gently stir beans, and return to oven. Bake uncovered until cooking liquid thickens into a sauce. Season to taste with salt and pepper, and serve with brown bread if you like.
Recipe Courtesy of Saveur Magazine Cookbook: Saveur Cooks Authentic American