Some black chalk, ground up, would do some tricks if you used it sparingly too Jayson. Be careful if you attempt any actual flame-work on the fringe/edges of his outfit to damage it. Some materials burn quickly compared to others. I'm sure it'd be possible though, to get a very realistic effect if you're very cautious.
Obi's robes were singed in the movie, but there were very few large "burns" on them. A couple here and there that looked like a hole had formed from some ash or something, but that was about it.