Since posting this, the more I think about it, the more I'm just saying "it'd be nice if WebExtensions were simpler to author and share."
But unfortunately, I feel that's kinda out-of-scope for Orion - the really correct way to do this would be to have a second 'kind' of webextension you can install, without bothering with authorization interface, without any sort of configuration, and that wouldn't clutter your lists of installed-extensions / permissions-per-website and so on. But, nonetheless, that's still "an extension", that can do extension-y, dangerous things, that should have the user's safety-hackles raised and eagle-eyes on.
My 2¢: close this as NYI, honestly. I can't see a user-friendly way to implement it that encourages good user behaviour and doesn't require buy-in from other browser-vendors that is never going to happen, lol.