Hi Kovid,
Thanks for the calibre content server suggestion.
Pros:
- avoids the registry keys/json stuff
- avoids additional download/install on top of the extension itself
Cons:
- would have to add the endpoint to calibre codebase. I personally have no idea where to start beyond there being an srv folder

- I don't think there is a way to do any kind of background web request with the extensions JavaScript API. So would have to launch another tab/window temporarily? Would need to test to see how annoying this could be.
EDIT: Actually after some investigation I think I could in fact use the fetch() function in javascript to invoke the url endpoint in calibre, so not an issue.
- user must have calibre content server running (I know this can be as easy as a checkbox to run at startup but I don't normally run it myself).
At the very least I can see it being a nice option to offer in the extension as an alternative to a user downloading/installing another app bundle.