the setting is saved as a file (nincfg.bin) that you must place on the root of your SD card :
sd:/nincfg.bin
redNAND shouldn't be an issue, the partition table only has one partition defined : the FAT32.
the only differences is that it's not starting at the beginning of the card, but at few GB away from it.
force widescreen : it doesn't affect the screen size displayed on your TV, it affects the game buffer width to be rendered on screen (more game info will be displayed, imagine you are using a wide camera in the game instead of a 4:3 camera), the game is still rendered 4:3 (640*480), except you see more game's element on your TV. it can affect the gameplay as some games are not designed to render "out of range" elements, like Paper mario. as a result, having a more wider area to display in 4:3 you lose some pixel's data (the game is still 640 pixel width, so to see pixels normally displayed outside this area you need to delete some pixel in front of you). as a result you have a "thin player", people looks like they are taller and thinner, circle are ovals (vertically). that image is then stretched by your TV decoding process on a Widescreen TV to fit all the TV area. vertical ovals are stretched to make the game proportion look good. aspect ratio looks good, but you lose details.
WiiU widescreen : it's used to toggle wide or 4:3 TV output (this is what you might be looking for).
on Wii, there's no option to render the picture in 4:3 and you need to modify your TV settings to display the picture in 4:3 (the default image ratio output by most gamecube games).
if you don't set the TV to 4:3 (with black borders on the side) then the 4:3 image output looks distorted on a 16:9 TV (circle looks like horizontal oval, people look short and fat). forcing the TV to display a 4:3 picture into a 4:3 resolution (with black bars) is making the game looks "pixel perfect" (circle looks like circle, not oval. People don't look Fat, etc.)
On WiiU, that option replace the need to touch your TV settings to add black bars on the side.
remember : black bars are good ! it's not a defect, it's how games were designed years ago. trying to remove them and get the picture to fit the screen and still look good is not normal, it requires compromises.
some games look bad with it (paper mario for example)
If you want your game to look "good proprotion" and fullscreen on a widescreen TV while the game is designed to be displayed in 4:3; you need to :
1. use a widescreen TV, with video set to widescreen (or fullscreen)
2. set "WiiU widescreen" option to be full screen (not 4:3, no black bars)
3. set the game to display a wider area (widescreen patch) if it doesn't have native widescreen setting coded by developers, so that when stretched to a wide TV it looks normal (circle displayed as circle)
4. if you still have (SMALL) black borders on the side, try changing the video width from 640 up to 720 to stretch the display area of the already stretched 4:3 output display a little more.
If you want your game to look "pixel perfect" (how it was designed to look) on a widescreen TV, you need the black bars!
1. either set your TV to display in 4:3 OR set the wiiu widescreen option to add black bars on the side.
2. do not enable force widescreen patch
3. keep the video width to auto (or 640)
For game with native widescreen option :
1. set your TV to display widescreen (fullscreen)
2. do not enable force widescreen patch, the game is already rendering a wider area of the game to "fix" the widescreen stretch looking on widescreen tv. (doing the same thing widescreen patch is doing, as the game is still displayed as 640px)
3. do not add black bars with wiiu widescreen
4. keep the video width to auto
5. set the game's internal option to widescreen to compensate the stretching effect done by your tv on a 4:3 buffer