Brine the pork: Combine 4 cups water, the salt, sugar, garlic, bay leaves, juniper berries, peppercorns and cloves in a large pot or wide Dutch oven (large enough to hold the pork). Heat over medium heat, stirring, until the salt and sugar dissolve. Add 8 cups ice cubes; stir occasionally until the brine is cooled.
Make small deep cuts, about 2 inches apart, all over the pork and between the ribs using the tip of a paring knife. Add the pork to the pot and submerge in the brine; add more cold water if needed to completely submerge. Refrigerate overnight.
Set a rack in a roasting pan and put the pan on the lowest oven rack; preheat to 325 degrees F. Remove the pork from the brine, pat dry and set aside to bring to room temperature. Remove 3 cups brine (along with the spices) to a medium saucepan; bring to a boil. Remove the hot roasting pan from the oven and carefully stand up the pork on the rack with the chine bone down. Add the hot brine to the bottom of the pan. Roast until the fat is mostly rendered, about 40 minutes.
Meanwhile, make the crust: Pulse the panko, butter, scallion, parsley, thyme and juniper berries in a food processor until the herbs are coarsely chopped and the mixture is combined.
Remove the pork from the oven and reposition so it is meat-side up; brush the top and sides with the mustard, then sprinkle with the panko mixture, patting to adhere. Return to the oven and roast 50 more minutes.
Increase the oven temperature to 425 degrees F and continue roasting until the crust is crisp and a thermometer inserted into the center registers 140 degrees F, about 20 more minutes. Remove from the oven, transfer to a cutting board and let rest 20 minutes before carving.