David Neevel of W&K Blog had been thinking about modding a guitar to use as a keyboard for some time. As shown in this video, he recently got down to business, interfacing the MIDI output through an optoisolator circuit to the Arduino. The interpretation of notes to keyboard characters is accomplished in code, with the Arduino then controlling David’s custom-designed relay board which interfaces to an old computer keyboard. Based upon the note played, the project sends the desired character to the attached computer. (Seems to us that the relays and keyboard interface hardware could be replaced in code, connecting the Arduino to PC via USB as an HID.)
You can find more details on this mod at W&K Blog, including project diagram and source code link.