In slow cooker, whisk together ketchup, sugar, vinegar, mustard powder, cayenne, 1 teaspoon salt, and 1/2 teaspoon pepper.
Add ribs to slow cooker, and toss with sauce. Arrange ribs, bone side up, in a single row. Cover; cook on high, 6 hours (or on low, 10 hours).
Serve ribs drizzled with pan juices and, if desired, barbecue sauce.
Oven method: Preheat oven to 350 degrees. In step 1, use a 5-quart Dutch oven or ovenproof pot with a tight-fitting lid; add 1 cup water. Cover; bake until ribs are tender, 2 1/2 to 3 hours. Proceed with step 3.