This would be the place to send messages: https://developer.mozilla.org/en-US/Add-ons/SDK/Guides/Content_Scripts/Communicating_With_Other_Scripts
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Small update on this.
Its possible to manually load it if you are desperate to use it:
I'm gonna try and monkey patch the CRX and see if that gets me into the store, otherwise i'm looking at a big rewrite/re-organize of the code which will take a bit longer.