Preheat oven to 325 degrees F.
Heat the butter in a large Dutch oven over medium heat. Add the bacon and onion and cook, stirring, until soft. Add the sauerkraut and stir to combine.
In a swath of cheesecloth, combine the juniper berries, peppercorns, caraway seeds, parsley, thyme, cloves, and bay leaves and tie into a bundle with a piece of kitchen twine. Add the spice bundle to the sauerkraut mixture. Pour in the wine and add enough water to cover the sauerkraut. Stir to combine and bring to a boil. Cover pan tightly with foil and braise in middle of oven for 2 hours. (The sauerkraut may be prepared up to this point 1 day ahead, cooled, uncovered, and chilled, covered with plastic wrap.)
Heat the oil in a heavy skillet over medium heat and, working in batches, brown the sausages.
Uncover the casserole and nestle the sausages, pork loin, and potatoes in the sauerkraut, partially submerging them. Recover and continue braising for 30 minutes more.
Serve the choucroute at the table in the Dutch oven or transfer to a large serving platter.