Pretty much just commenting to back-up what Chakratos has said. DDD works fine for this, you don't even need to enter an invalid IP. I only have a small collection of Japanese games but all 7 (one required a small bit of fiddling, the games fault... not DDD's) ran with no issues.
I've never tried doing this. Wouldn't be surprised if online play wasn't possible. Although, unlike games run in Loadiine, when you press the home menu using a disc booted-up using DDD, the actual name of the game appears (instead of MiiMaker etc).. and the games save to the Wii U itself. If you have success, could you let me know?
Because I like the JAP voices! And I also have a lot of mods for SSB4 that I use with smashexplorer and I can't get the undub patch + others mods at the same time so I want to mod directly the JAP version of the game