Transfer NNID & save data from one Wii U to another Wii U?

For several months I had been keeping my Wii U at my friends' house. They just got a new Wii U, and we want to transfer their NNID's (Nintendo Network ID's), purchased content, and save data from my Wii U to their Wii U while leaving all of my data alone.

We called Nintendo, and they said it is possible to transfer their NNID's from my Wii U (the source system) to theirs (target system), which will transfer the ID as well as any purchased content (it will be queued to re-download on their console). However, Nintendo's support site as well as the first person we spoke with says that for save data to be transferred, a complete system transfer must be done; this moves EVERYTHING, and formats the source system. Especially considering that I also have all my original Wii data on my Wii U, this is simply not a solution.

We have already initiated the process of transferring their NNID's to their console; we're just waiting to get the go-ahead from Nintendo to try to use them on their Wii U. So while we wait...

...this brings me to my question. The second person we spoke with suggested that it may still be possible to transfer save data. The process they proposed was...

• While logged in as NNID to be moved, copy save data from source Wii U to USB drive
• Copy all data from USB drive to another drive (e.g., computer)
• After NNID transfer is complete, insert USB drive into target Wii U
• If target Wii U reads data -- success! If not, format USB drive for target Wii U
• After drive is formatted, copy save data from computer to USB drive
• USB drive should now a) be recognized by target Wii U and b) have useable save data

And so we encounter our problem. Both Windows 8 and OS X Mavericks are unable to recognize any data on the USB drive and simply wish to format it. From what I read in a forum post from July 2013, Ubuntu Linux will at least see the drive, but not the data. It sounds to me like the Wii U is formatting the drive in some proprietary file system format, so...

• Has anyone figured out what this file system format is, and gained access to it?
• If so, will copying the data in the way described above allow their save data to be useable on the target Wii U?
• ...or is there some other method of doing this that we haven't considered?

Once their NNID's are transferred to the target Wii U (theirs) we will see if the USB drive is recognized by the console. I suspect it will prompt for a reformat and nothing else, which is why I'm posing this question to begin with. If for some magical reason it does recognize the data, then this whole question is moot, but in the meantime, any guidance is greatly appreciated!