It appears that you're travelling from the USA. You may be using Northwest Airlines. If that's the case, they do not have power jacks on the planes. Most planes do not have them anyway, as marsans said.
As for using your DVD player in Japan, there isn't a huge difference between the Japanese and the North American system, its a difference of 10v and most modern electronics are actually universal for the European market as well which I think is as high as 220v. America and Japan have 110v and 120v. So check the specs on your charger to be sure.