I love hazel. I have worked out a way of getting hazel to put pdf’s/doc’s etc into the global inbox, and even change status (for example apply labels).

However is there a way of using hazel within Devonthink ?
I know I could use (and do use extensively) smartgroups but hazel is so much more powerful.

Hazel’s not designed for usage in other apps. Got any specific examples of what you’d want to use it for? There might be other ways to get similar results, e.g. triggered scripts attached to groups or documents.