Wii u has not been hacked but the vWii (Wiiu running in Wii mode) has been.
there is no diskless exploit for it afaik so you would need one of these games....
1: super smash bros
2: Lego starwars complete saga
3: Lego indianno jones
4: Yu-Gi-Oh 5D's Wheelie Breakers
5: Tales of Symphonia: Dawn of the New World
you can run wii backup games and play gamecube games from sd card or usb hardrive but cannot play anything from backup disks.
I have never modded one but read up on it for a bit, with the intention of getting one once the wii U was hacked.
since it hasn't been hacked i sort of lost interest...
afaik, the console is much easier to brick...
so be carefull