To be fair, if it's not self powered, then even with Y-cable it probably won't work. Wii U is sadly messed up on the USB side, it doesn't supply 500mA of current on USB ports like is in the standard, but less (sadly, can't remember how much). So to run such a HDD you'd need a powered USB hub.
As for your main question - as far as I know nobody did anything like this yet. It basically would require a cold-boot CFW anyway, as WiiU throws a fit as soon it starts it there are multiple USB devices.