I believe the key search term was NSExtensionJavaScriptPreprocessingFile, e.g. documented here: https://developer.apple.com/library/archive/documentation/Ge...

That's cool, but also requires using the Singlefile extension (and granting it access), right?

What I like about the share sheet JS method is that it doesn't get access to most of my browsing sessions. (The shared-to app getting access to my browser session is somewhat unexpected, though.)