Hi,
I've been looking to do something similar, not only to categorise pages but content from other apps too (news, events and the like). For our purposes tagging was too flexible and prone to human error - we needed something more tightly managed and categories seemed the way to go. After some searching around I found washingtontimes django-categories (
https://github.com/washingtontimes/django-categories), which seems to do the job nicely.
However, while I easily managed to integrate it into my own apps (it displays a nice dropdown or multi-select of categories to choose from), I'm at a complete loss as to how to integrate it into django-page-cms.
Worth a look, and if you're any more advanced with django/page-cms than me maybe you can figure out how to integrate it. Or maybe someone else can help?