I have successfully written my first Safari Extension. It loops through all input elements and removes "autocomplete" attributes from the type="text" elements. This overrides idiotic websites which turn off autocompletion (e.g. remembering usernames and passwords).
I define the Whitelist of my extensions (in the developers tools) to be the websites I want this to work on. A major disadvantage - which is stopping me from publicly releasing this extension - is that every time I want it to apply to a new website, I have to add it to the Whitelist in the development tools and reload the extension.
Is there a way to programmatically edit the Whitelist or expose it to the users (and not just the developers)?
Turns out that this is not possible. You will need to login to the Apple Developer Forums to read the discussion there:
https://devforums.apple.com/message/660858
which resulted in filing an RFE with Apple.