I don't think they've outright said what group is working on Wii U.
However, I didn't mention a few of their other in-house branches, such as SRD (Systems Research & Development), IRD (Integrated Research & Development), and more. They likely handle more of the hardware load.
With 3DS and Konno at the helm of it, it was said at the time that Konno's involvement wasn't expected. It was the case of the higher-ups (Iwata? Miyamoto?) wanting to have a software dev's perspective on hardware.