As Chintu said, I can't see anything like that being stable. I can think of a couple of ways to almost simulate a mushroom cloud, but it'd look really gross and something like that isn't a flame I would be proud to have. I guess it all depends on how much quality you'll settle for. Perhaps PM'ing someone who's really good with the flame forge would help you figure it out. I don't know a lot of people that make flames, but a few of my friends have gotten some really nice flames from Furryprints.

Fin