In a large, enameled cast-iron casserole, cook the ham hock fat or bacon fat in the vegetable oil over moderately high heat until the fat is crisp, about 5 minutes. Discard the fat. Add the onions, jalapeños, garlic and ancho chiles and cook over moderate heat, stirring occasionally, until softened, about 8 minutes. Add the reserved ham or bacon meat and cook, stirring until golden, about 3 minutes. Add the barbecue sauce, tomato paste, the reserved ham hock bone and all of the drained beans. Stir in 4 cups of the reserved bean cooking liquid and season with salt and pepper. Cover and cook over moderately low heat, until the beans are tender and the gravy is thick, about 1 1/2 hours longer; add more of the bean cooking liquid if the beans seem dry. Discard the ham hock bone and transfer the beans to a large shallow baking dish or individual crocks.