Is it possible to restrict editing a PDF but still allow pages to be extracted and placed into a different PDF?

Not at the moment, no. The --disallow-modify flag also seems to disable extraction. Admittedly, if you can extract the pages and place them in a different (modified) PDF, you are basically modifying the original PDF.

I agree with that. If they really want to edit it, they will be able to do so since we allow printing. I just want to limit the ease with which they can modify the PDF because we want to keep track of changes in our system and they should use the HTML editing capabilities. The problem is that some users do require being able to place the pages into another PDF that we don't generate.

I'm wondering if it is even possible to do this with a PDF at all. If so, could this be added as a feature request?

Yes, it looks like this is possible. We will investigate it.

Actually on closer examination, it does not appear to be possible: the minimum level of PDF security is to disable page extraction, it is not possible to enable page extraction while preventing other kinds of editing.