Combine water, 1/4 cup brown sugar, kosher salt, and beer in a bowl; stir until sugar dissolves. Place the beer mixture, peppercorns, allspice, thyme, and pork in a large zip-top plastic bag. Marinate in refrigerator for 8 hours or overnight, turning occasionally. Remove pork from bag; discard marinade.
Preheat oven to 325°.
Combine 2 tablespoons brown sugar, ground pepper, 1/4 teaspoon salt, and mustard in a small bowl. Evenly coat all sides of roast with mustard mixture. Heat a large nonstick skillet over medium-high heat. Coat pan with cooking spray. Add pork, and cook 2 minutes on all sides or until browned.
Place pork on a broiler pan coated with cooking spray. Bake at 325° for 1 hour or until a thermometer registers 160° (slightly pink) or until desired degree of doneness. Place pork on a platter; let stand 10 minutes before slicing. Thinly slice; serve warm or cold.