3DS Homebrew to turn your 3DS into a USB game controller or touchsreen/absolute-positioning-mouse (Teensy code)
Requires 3DSUSBController-Homebrew and 3DSUSBController-Server
To run, follow the instructions at 3DSUSBController-Homebrew
Compile and upload using Teensyduino (Teensy)
I used a Teensy++ 2.0 without issue
As of 7/25/22, this repo includes a sketch (3DSUSBAbsoluteMousePositioning.ino) to use your 3DS touchscreen as a touchscreen for your PC (similar to a graphics tablet)! However, because of limitations of the Teensy itself, this sketch only works on Teensy 3.x and LC boards (source). Because I only own Teensy++ 2.0 boards, I cannot confirm if it actually works. I have confirmed that the code at least compiles without error. Please create an issue if you own one of these boards and it does not work for you.