Try a Polycell chemical paint stripper. You daub it on with an old paint brush, leave it for the recommended amount of time, then wipe or scrape it off. That might do the trick.
I think I would go for the paint stripper. It'll probably take more than one go though.
Or even a blowlamp to burn it off, then a good scrub to clean off the soot.
acp, I know that you asked this question a long time ago, but I wondered if you had any success at getting the paint off? I've been faced with the same problem, and I've found a system that seems to work. Can you come back and tell us if you have cracked the problem (or not)?