Add the pork belly to a large bowl or baking dish.
In a large bowl stir together the hot water, salt, cloves, and allspice, make sure the salt is completely dissolved. Pour in the cold beer. Make sure the brine temperature is below room temperature to avoid prematurely cooking the pork.
Pour the brine over the pork. Cover and refrigerate for 12 to 24 hours.
Preheat oven to 300°F
Remove the pork from the brine, add to a stack of paper towels, rind side down for 10 minutes to dry the skin.
In a small bowl stir together the kosher salt and egg whites until well combined.
Place a wire rack over a baking sheet that has been lined with aluminum foil. Place the pork on the rack, rind side up.
Pack the salt on top of the rind until completely covered and slightly compacted.
Bake for 3 hours. Remove from oven, raise temperature to 400°F. Gently pull the salt pack off the pork. It will be a hard crust at this point and should remove easily in a few large chunks.
Brush off any remaining salt with a clean kitchen towel or paper towel. Brush with olive oil.
Bake for 20-30 minutes or until the rind is a dark golden brown.
Stir together the maple syrup, 2 tablespoons beer, and mustard. Brush the top liberally with glaze.
Turn the broiler to high. Broil in the top 1/3 of the oven (a few rungs down from the very top) until the glaze has caramelized. Re-brush every 5 minutes. Broil until caramelized and dark.