Mitsubishi did make use of slave labor. Evidence is that the companies like Mitsubishi asked the Japanese army for the use of slave labor. Sendai Number 3, known as Hanawa, was one of the copper mining camps owned by Mitsubishi Mining. It has been covered extensively here to give an idea about the life at a Mitsubishi camp during WWII. There were at least 3 other camps apparently exclusively for Mitsubishi's use.
POW Claim Against Mitsubishi
Essentially what POWs are claiming is that the companies like Mitsubishi, who made them work on war-related activites (against the war conventions), didn't pay them for their labor.
The Japanese government required the companies like Mitsubishi to pay the POWs wages. MItsubishi and others made monthly reports to the government as if they were paying the POW slave labors.
But they were not paid. The government also expected Mitsubishi and others to provide housing, food, sanitation, heat, but the POWs were very almost starved, without much medical attention, and extremely poorly housed.
All the POW slave labors are asking monetarily is their back wages with interest. Is this too much to ask for?
Recently Mitsubishi rejected a Chinese slave labor lawsuit demand by saying it bore no responsibility since it was national policy to employ Chinese laborers. Will be analyzing it later, but what about the American POWs, many of who had to work for Mitsubishi companies under unhumane condition? According to Linda Goetz Holmes in an interview: "What struck me when I was doing the research was that the companies immediately asked for the use of the prisoners. It wasn't something the Japanese army decided to do with all these men they'd taken. It was the companies that asked ..."
List of Mitsubishi (in bold) companies out of 25 involved in slave labor lawsuits filed in the USA as of 05/12/2000 (source AXPOW):California Portland Cement Co.