Finally someone made something like SaveGame Manager GX for Wii U, fantastic!
After you implemented the multi user feature you could also look into Loadiine compatibility.
Switching savegames between Loadiine (SD:\wiiu\saves) and sysNAND/USB is the only reason to use Saviine for me.
A note on savegames between Loadiine and NAND:
Every user seems to have an unique ID that starts with 8000000x and the X seems to be a random number for each user. In my case it's 4 for example and I just found out by dumping a random game's save with Saviine.
So to implement Loadiine compatibility you just have to "map" those folders (as it has to be done with Saviine right now):
SD:\wiiu\saves\Game [ID6]\c
is the common folder for NAND saves
SD:\wiiu\saves\Game [ID6]\u
is the user folder (8000000x) for NAND saves
I dunno how you could let your tool figure out that x (maybe just by dumping something) but once it has it it can move saves between NAND and Loadiine folders on SD without problems I guess. Hope that helped.