mrjeeves Let's try something here...
Disclaimer - I'm suggesting this without my giff-staffer hat on, so as one member to another, at this stage.
I've pushed the base extension code, along with the definitions I gave earlier to a public repo, here. If you're happy, feel free to push the CSS for the checkbox to that repo, and see how we get on.
If all goes well, we can look at moving out of this repo, and into a giffgaff one, when we have all the working arrangements, etc in place. How does that sound?