There's a way, but it works only for Wii backup loaders on FAT32/NTFS. (not a WBFS partition)
No other homebrew, no NANDEmulation (if located on USB).
It's hiding the partitions by altering the partition's table, so the HDD will NOT work on your computer anymore.
So, it's a little dangerous, you could loose your data if not done correctly.
We didn't test writing files and installing games yet. but playing is working.
With this method, you can keep two HDD connected at the same time, one for vWii and one for WiiU.