Source: https://github.com/vgmoose/vgedit
vgedit+ (Video Game Text Editor) is a basic SDL2 text editor designed for easy use on a video game console using a controller or touch screen. The primary goal of this project is to allow you to not have to turn to another device to edit text files.
This was originally designed for the Switch, but as it uses the Chesto UI library it also runs on Wii U! Hopefully this homebrew will help you in a pinch, or just save time going back and forth with a computer. It supports a USB keyboard and should be able to edit even large text files on the SD card. One example quick use-case of this app that I've used already is customizing the Haxchi config.txt before installing. It probably pairs well with WiiU-Shell.
Screen shots
Controls
Touch screen also works on all buttons / scrollable areas or to position the cursor in handheld mode
An external USB keyboard should work for USA-keymap typing, and arrow keys to move the cursor
File Browser
A - Open File
B - Up a directory
X - New Folder
Y - New File
Minus - Exit
While Editing (Overview mode)
A - Show keyboard / start typing (enter Insert mode)
B - Delete under cursor
X - Copy
Y - Paste
L - Find / Deselect selection
R - Increase selection
ZL - Undo
ZR - Redo
Plus - Save
Minus - Exit
With Keyboard up (Insert mode)
A - Select letter
B - Backspace
X - Caps toggle
Y - Stow keyboard (back to Overview mode)
Alt Key (via USB) - Hide on-screen keyboard (but stay in Insert mode)
Pontifications
You can view the stats from hb-appstore for vgedit+ downloads across Switch vs Wii U here: https://apps.fortheusers.org/stats?apps=wiiu/vgedit,switch/vgedit (last 14 days). I find it a lot of fun to watch the same homebrew app on both consoles race like this!
My advice having worked on this text editor is: I don't recommend making your own text editor! It seemed a lot easier of a problem than it turned out to be! (Line wrapping, large files, cursor positioning, tabs?!?)