The only way that I know of is to email some of the developers (I know a few people who have done this) and they'll let you change it. However, it's rare that they'll listen to you.
There are plugins for bukkit that change how your name appears within the chat but seeing that I've never used them, I don't know if they work for the tag above your head. This obviously isn't a good answer but it is the only thing I can think of at the moment. From what I've heard, the game relies on those tags by account name so if that name is changed, it will end up kicking the person from the server or something along those lines. This works for just about any "sentient" entity, the tag is just invisible for mobs. As for this entire post, it's pretty much just a tangent.
I ment to put in InfernoHero222, but I put in infernohero222 by mistake.
Don't make things too complicated.
i hope they include this.
Unless they make it cost.
THE TRUE LORD; FATHER NURGLE