Upscaling will not produce the same image quality as Dolphin does. Dolphin is not upscaling the image (Ie just simply resizing a 480P source to 1080P) but actually rerendering it to 1080P. I have attached two screenshots to illustrate the difference. The first was taken with 4x SSAA, the second one was taken with XFB set to Real. To get as close as possible as what the actual Wii does, I have also disabled EFB scaled copy, SSAA and any other enhancements.
Upscaling is already possible with the Wii using something like VDIGI VD-W3, but frankly it looks like ass.