CPU architecture is identical, bro. Not sure where you're getting "the Wii U has a Wii in it" from either. I did a quick google search and didn't come up with anything supporting it.
I must be remembering that incorrectly, then. Sorry 'bout that.
Unfortunately, Google also turns up that the Wii U's disc drive doesn't accept GCN discs (tools like Devolution require you to verify your discs on a normal Wii before you can load the backups on a Wii U because of this), so it'd take more than just a firmware upgrade.