Aplogies for repeating question, but I just wanted to make sure this was not possible :

Is it possible to intercept the event that checks authorization when attempting to edit a document (eg that returns error Authorization error: Unauthorized access to URL: if you attempt to edit a doc using the edit url if you don't have authority to edit )

and add my own checks (eg based on specific document ids, external user groupings etc)?

To clarify, I don't want to undermine Umbraco authorization, I want to add extra layers of security so that even if a node is within an editors tree of nodes (ie they can see and access it normally) I want to be able to flag certain documents so that they can't be edited. I know I can intercept the save (and hopefully delete/publish/unpublish) events but it would be even better to prevent access in the first place to back office editing ie generating the error above.