Clean it out really good first. That should handle most of the problem. Lacquer thinner does not leave much of a residue, so if you can't smell it after cleaning its probably close to being okay. Then burn it for a while. Although I wouldn't start a fire in it if I could still smell lacquer. That could end badly, because of the explosion possibility, but also you don't want to inhale any burning fumes.
As long as its cleaned thoroughly and burned first, it should be fine. The food should never come in contact with the sides of the barrel anyway.
I would leave the "lacquer thinner" label on it along with the hazard symbols. It will make the smoker look that much cooler.
ps: I read a label for lacquer thinner awhile ago, it said it is "know in the state of California to cause cancer". So as long as you are not in California, you should be okay. :wink: