Extortion. "You keep using that word. I do not think it means what you think it means."
I am certain your day job isn't law and I would recommend you not give up your day job any time soon. Mt.Gox failure to disclosure their identification policies in advance may be shady hell it may even be illegal (doubtful) but under no possible interpretation of any law is it extortion. Even if it WAS extortion that is a criminal matter. You can seek civil damages for a criminal charge.
Yes, extortion is a criminal act but "coercion" is a type of extortion. In this case perhaps I should have just stuck with coercion. And you can certainly get civil damages for financial coercion. Regarding civil damages for a criminal act, you can sue as a victim of a crime and you can get damages. Just google "Civil action for damages for criminal act" and click any of the links on the first page for a variety of state code on the matter.