Step 1: Place the ginger beer, lemon zest, and cinnamon stick in a heavy saucepan. Boil over high heat until reduced by half.
Step 2: Add the ketchup, sugar, rum, molasses, Worcestershire sauce, lemon juice, liquid smoke, onion powder, and pepper and whisk to mix. Gradually bring the sauce to a simmer over medium heat, and continue to simmer until thick, 10 to 15 minutes, stirring occasionally. Correct the seasoning, adding brown sugar or lemon juice to taste: the sauce should be very flavorful.
Step 3: Transfer the sauce to a bowl or clean jars and let cool to room temperature. Refrigerate until serving. Refrigerated, it will keep for several weeks.