How to Make It
In a large pot, combine all of the ingredients except the pork belly and simmer, partially covered, over low heat for 30 minutes. Let cool to room temperature. Add the pork to the brine and leave at room temperature for 1 hour, then cover and refrigerate for 24 hours. Bring to room temperature for 1 hour before cooking.
In a small saucepan, cook the rhubarb juice over moderate heat until reduced to 1 cup, 8 to 10 minutes. Add the maple syrup and cook until slightly thickened, 5 minutes. Stir in the fish sauce and chile powder; let cool.
Light a grill and set it up for indirect cooking. Remove the pork belly from the brine and score the skin in a crosshatch pattern. Set the pork on the grill, fat side up, opposite the heat source. Cover and grill at 325° for 2 1/2 to 3 hours, until very tender; brush with the glaze during the last 45 minutes of grilling. Transfer the pork to a cutting board and let rest for 15 minutes. Thinly slice and serve.