I don't know enough to say this absolutely isn't possible on Wii U, but I'm prepared to go as far as saying it's unlikely. The thing with the Switch is that it's very much designed around running at different clock speeds: there's the handheld speed, the docked speed, and even the boost speed to reduce load times. "Overclocking" the Switch is really just taking advantage of a feature the Switch already has, adjusting clock speeds for different uses. The Wii U doesn't really do this so much. While it does downclock to run in Wii mode, as far as I know Wii U mode just sits at max clock speed all the time, so there's nowhere else for it to go.