NAS= network attached storage. Basically a little device you plug onto your network and it provides various flavours of network shares.
If your router has a USB slot you can plug things into and share files it is basically this.
You can also turn an old PC into such a thing if you wanted.
Anyway I have not kept up with Wii U media players, however I would be surprised if there was not some mplayer port that had FTP, SMB and some of the others by default. If your NAS is a fancy one you might even have some form of transcoding available which could help (I would not expect my Wii U to usefully decode H265 but H264 should be within reason) with some things.
I don't think there are any versions of XBMC/Kodi worth speaking of if you have that as a baseline though
I used to watch some mp4 movies (no other video format works afaik) directly on the Wii U Browser by configuring an http file server on my phone connected to a HDD via usb otg. It was kind of a hassle but it worked.
Unfortunately there is no homebrew video player available in Wii U mode, just WiiMC in vWii.
Unfortunately there is no homebrew video player available in Wii U mode, just WiiMC in vWii.
