I would either steam it or head to the car wash. You can try burning it off but unless you get the WSM up to 450-500, you may wind up baking it on and making it twice as hard to get off.
I'm a BIG fan of Dawn Professional 56037 Dish Power Dissolver. It isn't as strong as oven cleaner for burnt on food but works like magic on smoke residue. I can squirt some on a foil pan that's been in the smoker for 5 hours, take a soft brush to it and everything rinses off.