I've tried the coffee can method and didn't notice a longer charcoal burn, and I also don't like the idea of a bunch of unlit briquettes exposed at the top of the fire. The biggest issue with the '09 and newer 18.5" wsm is charcoal capacity, since there's little distance from the pan to the fire. So to get a long cook, you need to pile up the charcoal in the ring as high as you can and still have clearance for the pan. Easy to do without a fire, so obviously, you can do that best by lighting with a propane weedburning torch and not having to worry if all the lit will fit under the pan.
Also, you can swap the Weber water pan out for a Brinkman water smoker charcoal pan to add a lot more clearance since it's bottom is flat. (Don't worry. It holds almost as much water as the huge Weber pan does.) So basically, I recommend either doing that or lighting with a weedburning torch for long cooks....and it wouldn't hurt to do both since charcoal useage can go up tremendously if you have a combination of lots of meat on both racks and less than ideal weather. Add water to the pan and you'll use even more, and that's something to think about as well. There'll be plenty of moisture in the pit if there's meat on both racks to prevent your bark from getting too dry or thick, as long as you manage your fire properly. HTH.