I know that isn't what you meaned but you can play all Wii games on the gamepad: It contains a sensorbar. So The Wii U gamepad + a Wiimote is a semi-portable Wii...
You'e able to remove that restriction. I can't find a link right now but there's a small regulator electonic near the USB ports. Easy to bypass with basic soldering skills. The most simple unlimiter mod i've seen so far is this but note that this is technically not bypassing the restriction but soldering a Y-Cable directly into the Wii U. So both ports share their energy:
Another one is to connect these 3 dots, that one bypasses the regulator completely: