Hi Martin,
If you're just looking to build something for fun, I would say that this is probably too tricky to be worthwhile right now. If you have a more serious use case, I'd encourage you to fill out the form linked in that blog post, even if you don't strictly fall into the audience that form was targeted to. Google Docs has some functionality where allowlisted extensions can enable an "annotated canvas" mode that provides more information in the DOM. My understanding is that they regularly reach out to respondents of that form to offer access and more information.
Thanks,