All you have to do is trim the fat and increase the performance of the memory management of the engine of the Wii U, by reducing things that sit of the ram which can be easily done. First what is rendered smaller, as in the loaded size of the stage, by either making the game more modular or if you are using dynamic loading reducing the draw size. Also you can prioritize textures and other assets if its not being used load it out, or if you can eliminate it. The developer who get a hold of how to program for the Wii U before the other systems go into full swing will have no to little trouble doing this since they will have a working engine that can do that. And if they are using a caned engine most of the work is done for them.
You can also go the route of Lego City and require a Hard Drive to play the game. It may cut your potential user base but then you also can use the hard drive to cache much like they had to do with PS3.
This is very simular to asking how a game can work on a system with only 1GB of memory availbe but is made to run the most effectivly on a system with 7GB of RAM, you turn off or dial down memory eating resources. The PC market has been doing this for decades.
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