If he were to team up with people who are U.S residents, does that pose any problems?
I'm assuming if the team wins, he gets the money, but then it is on his honor to distribute the funds to the rest of the team? I'm assuming the App/system should be registered with his challenge post account, or is it okay, if it is registered with a U.S resident challenge post account, but he (the Japanese working visa resident) is identified as team member?
@Peter T remember there are many prizes to compete for. The award is a Japanese prize to promote the developer community there. It seems that since your team is primarily US residents - perhaps focus on the general Devcup prizes to be safe.