Google Spreadsheet Based Version
The idea for entering assignments quickly into a gradebook using QR codes originally occurred to me soon after I started using a Google spreadsheet to create a Gradebook Web App to keep track of grades for my students. Example code and instructions can be found here:

Moodle Activity Plugin
As I started using Moodle more and more, I realized I could write a plugin to accomplish the same functionality. Being my first plugin, the GradeQR-MoodleActivityPlugin is a little rough around the edges, but it is functional and it is already saving me a tremendous amount of time. I am currently working on submitting it to Moodle's plugins directory.

Moodle Submission Plugin
While the activity plugin serves my purposes, it would be much better if this concept were integrated into the assign activity plugin as a submission plugin for a variety of reasons. Unfortunately, I am having difficulty with this project and am in need of help. See my GradeQR-MoodleSubmissionPlugin repository for details.