Here's a bit of good news. One unlucky chap had a bricked Wii U. He sent it off to Nintendo and wound up getting a new unit in return. When setting up that new unit, it asked for his old NNID, email and password. This transferred his account to the new Wii U, but it let him know that he couldn't do this with another system. Looks like Nintendo can set you up with your old NNID in certain circumstances.
Unfortunately, this Wii U is also having issues, so he might have to go through the process all over again.