Prep: Bring water, sugar, and salt to a simmer in a large pot, stirring to dissolve. Remove from heat. Add ice, bay leaf, thyme, orange zest, and peppercorns, and let cool completely. Place pork in a nonreactive dish, and completely cover with brine. Refrigerate at least 6 hours and up to 8 hours. Remove pork from brine, and let stand at room temperature 30 minutes. Pat dry, and season with salt and pepper.
Grill: Heat grill to medium. Grill pork, covered with vent open, turning every 6 minutes, until internal temperature reaches 125 degrees, about 32 minutes. (Pork will continue to cook to proper doneness as it rests.) Transfer pork to a cutting board, and let rest 15 minutes. Slice pork, and squeeze orange over slices. Serve with mustard.