Preheat the oven to 325°F. Rinse the barley with cold water until it runs clear. Place in a medium ovenproof saucepan with the stock and ½ teaspoon salt. Bring to a simmer, cover, and bake for 35 minutes. Remove, rest for 10 minutes, and fluff with a fork. Transfer the barley to a tray, spread into a thin layer, and chill uncovered in the refrigerator.
In an electric mixer fitted with a paddle, mix the butter until creamy. Add the cooled barley, the breadcrumbs, mustard seeds, mustard powder, Dijon mustard, and grainy mustard and season with salt and pepper; mix just until combined. Scrape the butter onto a sheet of parchment paper, set another paper on top, and roll into a ⅛-inch-thick sheet. Refrigerate until firm, or for up to 3 days.
For the Pork Rack
In a large saucepan, simmer 2¼ cups water with 2¼ tablespoons salt and the brown sugar until dissolved. Remove from the heat, add the mustard seeds, cracked peppercorns, and half of the sage, thyme, bay leaves, and garlic; allow to cool. Place the pork in a 2-gallon resealable bag and pour in the water–spice mixture and the beer. Seal and marinate refrigerated for 48 hours, turning the pork 4 times.
Preheat the oven to 300°F. Remove the pork from the marinade, scrape off any herbs or spices stuck to the meat, and pat dry. Season on all sides with white pepper. If desired, wrap the bones with aluminum foil to prevent browning.
Heat the oil in a roasting pan over medium-high heat. Add the pork and sear on all sides until golden brown, about 8 minutes total. While searing, baste often with the oil from the pan, especially in the areas around the bones. Reduce the heat to medium and add 2 tablespoons of the butter and the remaining sage, thyme, bay leaf, and garlic. Continue turning and basting for 3 minutes.
Transfer to the oven and roast until the internal temperature reaches 130°F, 45 minutes to 1 hour.
Meanwhile, bring a large pot of salted water to a boil and boil the cabbage leaves until tender, about 4 minutes. Strain off the water and add the remaining 1 tablespoon butter. Toss with the horseradish to heat through. Season with salt and pepper.
Remove the pork and increase the oven temperature to broil. Remove the barley crust from the refrigerator. Press the crust onto the meaty side of the pork and trim any overhanging edges if needed. Broil the pork for about 5 minutes, until the crust is golden brown.
Arrange the cabbage on a serving tray and set the roasted pork rack on top.