I did some of my own research, I found very little on it. The only reason I asked because in the film Unbroken the Japanese media try to convince Louis to join other Americans in speaking ill of America to escape from the war camps. It is not elaborated in the film what happens to them and as the film is a true story I wanted to find out more. In my research I only found one example of an American who broadcasted for Japanese radio stations during WW2, Tokyo Rose, her real name was Iva Toguri D'aquino, who was an American citizen who went to Japan to take care of a sick aunt. However she got stuck there when Japan bombed pearl harbour. As an American she was an enemy of Japan, she was forced, as she could speak fluent English, to broadcast against Americans fighting in Japan calling the fighting Americans our enemies. She made an effort to get food to POW which was successful. However in 1949 she was jailed for 6 years for treason, pardoned in 1977 by President Gerald Ford and died in 2007 at the age of 90. If there were any other people who went through a similar situation as a POW I imagine they were pardoned, executed by the Japanese or jailed by the US. This answered my question to the extent I was looking for.