Probably when it is finised and all the little problems are worked out.
But you would basically do the same as with SDUSB, the only difference is that you parttion the USB device instead of the SD card. Make sure it is MBR, not GPT, the FAT32 Partition should be the first partition and the one you want to use with the Wii U the physically last one. Of course backup your stuff before. If theplugin doesn't work for some reason, the whole disk will be formatted.
Also the FAT32 partiton on the HDD can't be used on the Wii U side right now, and I am not entirely sure if it will be possible or make sense to use it. It's like to require some update to Aroma to propery support it and even then Coldbooting Aroma might not work from the FAT32 partition