Would it be possible to be able to configure additional steps before (and potentially after) the URL is opened, ideally to include "Run Script"? Like Hazel but for URLs.

The URL could be sent as a parameter to a script and its result optionally used to replace the URL.

This would permit (depending on the script):
- the URL to be rewritten
- a particular Keyboard Maestro macro to be run
- a particular browser window to be opened (all links from Mail open in a single window, say)
- swap to a particular desktop first (and use the version of Safari on it)