Well, I just wrote to Nintendo to ask what the hell I should do. I'll try real hard to make this simple:
My parents bought a Wii U at launch. I created an NNID on it, but NNIDs are linked to the console. I moved with my boyfriend, we got our own Wii U, I'm stuck and can't access the games I bought on my parents' console. I called customer support last summer and asked for an NNID transfer, they said there would eventually be an application to make transfers easy in the future. I left it at that.
Now, I really want to play the Bravery Default demo with the bonus stuff, and of course use the Pokébank too, but now it's mandatory to have an NNID on the 3DS to download from the e-shop. I would link my existing NNID, but if I can never access it again on Wii U, it would suck if I had different NNIDs on my 3DS and Wii U. But at the same time, I don't really want to create a new NNID (and hence a new username, ugh...), start over all my Miiverse posts, and never have my games again (virtual console only, but still...).
So I just wrote them all this, asking what I should do now. I really hope they will answer soon, and with an answer that will let me keep my old stuff... Let's see how it goes.