Easy Off oven cleaner.
I had the need for this very thing last weekend when I smoked a pork butt in my water smoker. I broke rule #10 and paid the price.
The meat turned out fine, but my water pan dried up and produced a thick, black crust from the drippings. I scrubbed with a green pad, but I still couldn't get it off completely. My wife shot the pan with the cleaner while I was at work the next day, and she saved the day. It worked like a charm because the water pan is porcelain.
Just remember to rinse thoroughly after cleaning. You don't want to cook with that nasty stuff.