Thanks! I thought this was impossible. Gonna try it out today.

Edit: I successfully signed the key on AMO. Here are the steps:

    Get an access token from https://addons.mozilla.org/developers/addon/api/key/
    In manifest.json, add browser_specific_settings.gecko.id and set it to something like "myext@example.com"
    Run command: web-ext sign --api-key=<jwt-issuer> --api-secret=<jwt-secret> --channel=unlisted
    That command will upload your extension to AMO. After an automatic review, you can download the .xpi file from AMO.

That said, it's not ideal for me since I make extensions for work. Looks like a human reviewer can check your code at any time.