I could look at this topic, historically, as being equal to charging me with the Oklahoma City Bombing. I wasn't there, had no conversations with the bombers, didn't even know the bombers, hadn't heard of them, and had no part in the decision. So while a mock trial is an interesting historical exercise, I can't see a way where MacArthur could ever be charged with this crime. Truman perhaps could be. It was his decision.
By following pohnpei's link above, you'll notice that by his own admission, MacArthur was not consulted about the decision, nor did he think it was the right one. So it would be a very difficult case to prosecute, with no signed orders, conversations or physical evidence that he ordered the bombing, because he didn't. It would be a very easy case to defend because, ladies and gentlemen of the jury, my client wasn't there.