If I understand Nintendo's stance on Wii games on the Wii U, they won't be up-scaling them because they can't. They're most likely in the same boat as Sony was with PS2 games on the PS3, which involved not emulating, but packing in the prior generation hardware into the unit to have that backwards compatibility. It couldn't go beyond up-sampling as that was a limitation to using real hardware vs emulation, whereas something like Dolphin doesn't have that limitation due to doing complete emulation. In all likelihood, Nintendo is going the hardware way with Wii games.
If they were to go the emulation route, then I wouldn't see anything stopping them from up-scaling to HD quality.
The reason BC PS2 games had to use low-res is not because they used the older hardware versus emulation, but rather because the PS3 video hardware was 100% incompatible with the the PS2's video hardware. It wasn't the PS3 video hardware that rendered PS2 graphics, but rather REAL PS2 hardware had to emulate the PS2 graphics because the PS3 graphics were completely incompatible. This is true for PCSX2 (the PS2 emualtor on PC). The video hardware in PCs is completely incompatible with how the PS2 renders graphics, meaning the entire PS2 Vector Units have to be fully emulated via CPU processing power, and then just the final leftovers handed to the PC graphics cards to display the final image. That's also why PCSX2 has so much higher system requirements than Dolphin, even though the PS2 had lower end hardware than GCN or Wii.
The GCN/Wii/Wii U graphic hardware, however, is actually a natural progression of fully backwards compatible hardware. All three graphics chips were designed by the same company (ATi/AMD) and are more or less just console versions of PC graphics cards. These wouldn't need to be "locked" into a certain hardware configuration, nor would the Wii U require having a secondary graphics chip that's designed just to play Wii games (See early BC PS2 models)
Your example is like saying I couldn't play an old 3D PC game designed around 800x600 displays properly rendered at 1440x1080 because the games aren't being emulated, but rather just being rendered on newer compatible hardware.
That said, Nintendo could just as easily not upscale for any number of reasons (the biggest being thoroughly testing the entire library, though this could be remedied with a small warning about possible graphical glitches when playing in 'upscaled' mode). I would hope however that nintendo would do proper upscaling so I wouldn't have to rely on Dolphin's often wonky compatibility/glitches just so I can play Skyward Sword in HD.
(edit) Forgive me... rereading this blob of text even I had a hard time gathering my meaning. That's what happens when you write something at 10am and you haven't slept in 2 days. Shortened version below:
The PS3 running PS2 games is like the Super Nintendo running Gameboy games (via the Super Gameboy)... That is, the games aren't being emulated or run natively through themselves, they are literally just two systems put together.
The Wii U running Wii games is neither emulation OR two seperate systems stitched together. It's one set of advanced hardware that can run the older software. It's like playing Quake 2 on my current PC. Quake 2 wasn't designed to run on my state of the art PC, but it still plays on it natively and therefore can benefit from the enhancements available to it.
The question still comes down to whether nintendo would or not.
(edit 2) snipping out some of that extra stuff.