That would be an entirely different undertaking - it would take two things. A program to boot vWii in 480p native graphics without manually switching, and a modified CFW to automatically set the Wii U back to 1080p upon reboot. This program is just a proof of concept if you think about it. Besides, while the Wii U's upscaling might not look the best (subjective), it would almost certainly be faster than letting your display handle it.
Thanks for the clarification, if you haven't tried it, you should before speaking on it as you'll see the difference clearly yourself. As for your other points:
The TVs upscaler in my (and quite a few other members cases) looks
objectively much better than the Wii Us crappy scaler as its a night and day difference and my display handling it is actually
faster than letting the Wii U process. Win/Win for me.
Only issue is having to change video modes back and forth is cumbersome :/
My thoughts were along the lines of editing the vWii side of things to switch the output as I'd read that all Wii U side things get unloaded when you're in vWii mode and that it must just be checking a variable flag to what its output is therefore it should be relatively easy to edit said files to make it output 480p exclusively in vWii whilst leaving Wii U at 1080p.
More than one way to skin a cat.