See, the thing is, though the Wii U does have these capabilities, as far as playing Wii games, it doesn't matter. I have some experience with this considering the Gamecube compatibility on the Wii. When playing a Gamecube game, there is no way for it to see the Wii hardware such as the SD card, USB ports, Wiimotes, any of that. Basically you're just playing a Gamecube as is within the skin of a Wii.
It's most likely the same with Wii software on Wii U. Though there's all this stuff that the Wii U adds, when running Wii software it's invisible and might as well not exist. This is also why you can't run Wii VC games on the tablet (the Wii can't see that so it can't use it). So much like the Wii-skinned Gamecube when playing, say, Mario Sunshine on Wii, so too will it be a Wii U-skinned Wii when playing, say, Mario Galaxy.
It's also the same thing as needing a PS1 memory card when playing a PS1 game on a PS2; the PS1 game can't see the PS2 memory card. It's not programmed to, and how could it be? You can't program something to be compatible with something, to be aware of something that doesn't exist at the time of programming.
So honestly, though having HD-upscaled Wii games would be nice, they would have to be re-released (much like they did with some games like Pikmin 2 on the Wii) in order to do so. As it stands, Wii games can't take advantage of the HD capabilities of the Wii U simply because they aren't programmed to. When the Wii U runs a Wii game, it's the same as running it on the Wii.