Dave, a good washing will get rid of any residual ash from whatever was burned out and yes, my first plan of action is to brush it down to bare metal with an angle grinder with a brush in it. Then I'll wash it out again and once dry, I'm thinking of giving it a coat of high temp primer, then a coat of high temp paint the same as the outside will be. With it being burned, washed, brushed, then coated with two layers of paint, how could I get any fumes inside the drum?
As far as a car wash owner getting pi$$ed off, by the time I take them to the car wash, they'll just be burned metal drums with no ID on them anyway. I'll hose them out at home first so they'll even look clean inside too. If I had a powerwasher, I'd do it all at home.
I only posted this question in hopes that someone might know something specific. I guess my opinion is that if it's burned out, washed and wire brushed, then sealed behind heat proof paint, it should be pretty safe by then.