This thread is still pinging my inbox, it's a great technique! For some reason I missed your question before.
I did it this way because the dome temperature is typically warmer than other places on the grill. It also keeps the roast closer to the water. I've done it both ways since then and I still like the inverted aluminum pan, inside a larger pan the best. Plus it's easy to get the roast to the table for pulling!
BTW, I still soak the wood chunks too. I may be paranoid now but the affect I described was what happened. With the dome in place, it would flame out and raise the dome temperature by a noticeable margin. Soaked no problem, just smolder and smoke.
Glad to see this thread still active. I made Woodburner's pork shoulder today, after experimenting with the fire yesterday. It was absolutely fantastic. Never thought I could pull something like this off on a Weber.