Citizen's arrest based on what? You need to tread carefully when doing such a thing.
No, it's simple as pie. If you have grounds to believe that there is a crime going on, and that the criminal may be about to run with the money, you just detain him and leave the matter to the police for investigation. The police cannot fine you unless they find out that Mt.Gox is committing nothing criminal. In the complaint you outline the crimes in such technical precision that the police cannot find them not true without real investigation. So they will have to investigate.
Of course they
can summarily decide that you are wrong and the arrest was unlawful, but if Mt.Gox ever is found guilty (which I would say is 99% sure at this point), you will triumph as a hero and Japan police and judicial system is shamed.
This is exactly why citizen's arrest prevails in the legal code. When it's taken away, then there is nothing more to do but whine.Sounds like a gamble.
You have to remember that laws in Japan likely will be different than where you are from.
Sounds risky if you turn out to be unable to prove the crime has taken place.