The short answer is, you can't. Nautilus is not built to allow that sort of thing and you can go through the source code of nautilus and look at the file copy/move sections to see how it works.

Part of this lack of integration was the motivation behind the progress indicator design. It would replace the nautilus progress window and several other progress inconsistencies and allow multiple programs to show their progress to the user in the same way:

https://wiki.ubuntu.com/Ayatana/ProgressIndication

More work and advocacy for these new designs would help, allowing integration of all sorts of tools.