Wait, do now disassemble the drive and hook up only PCB? Will it need power connected still? Insulate it to avoid contact with metals and hopefully have room for hard drive? Trying to follow your idea. If power is off maybe the Wii U still won't detect drive? Still the same as taking it out. Would work with cables still connected and PCB insulated. I am curious to know what Wii U does with drive only disconnected. Then boot? In menu? When going to run games from a hard drive without disc drive in? Basic trouble testing;-)
You're making it too complicated. Leave all cables connected to the pcb from the mobo and just disconnect the pcb from the disc drive. Remove the disc drive and place the hdd in it's place. No need to insulate anything as the pcb is not insulated, it just sits on the bottom of the optical drive. I would make sure the hdd pcb and the optical drive's pcb aren't directly on top of each other to avoid interference. No need to "disassemble" the optical drive as the pcb just sits on the bottom of it (should only be a couple of screws to unmount it and 1 or 2 ribbon cables connecting it to the drive).
In theory, this should "trick" the system into thinking the optical drive is still installed, providing it works the same as the PS3 checks. Hell, my eject button on my ps3 still acts as though a drive is connected even though it doesn't do anything. For all intents and purposes from the system point of view, the system still believes the drive is connected.
This is all in theory, mind you as I won't attempt this myself for something like this. If I actually had a way to connect the hdd to the motherboard and use it instead of the optical drive, that would be something I would be interested in and then I would be "onboard" so to speak for a project like this.