NAND Restore after Wii U transfer?

I assume you mean back it up on the Wii side.

You can do that easily, transfer all your content, and keep your saves and purchased content installed on both machines. It's a rather common thing to do around here.

In fact I would recommend it. Why? Because, as it turns out, my Wii U did NOT transfer all my savegames from my Wii when I did the transfer like it was supposed to. It completely ignored a bunch of my saves during transfer, like SSBB (actually Savegame Manager GX would not transfer my SSBB save either for some reason, even though the game can read it fine on my Wii) and I'm pretty sure it ignored a lot of other ones too as my vWii Data Management has very few pages of content on it compared to my Wii.

So if I didn't have that NAND backup, it might've deleted all the saves on the Wii (it isn't supposed to touch content it doesn't transfer, but then again it's not supposed to ignore half my save files either, so the result would be unpredictable) and I would have to start over in a lot of games.