Open Radar

 
In Pages, in Page Layout mode, you can 'capture' a page to make a predefined page type (which shows up in the New Page menu). You can go back and rename or delete previously captured pages, but that's all you can do to them.

I propose you should be able to edit Master Pages, similar to Master Slides in Keynote. The UI for making and editing them should work the same. You should be able to change the master applied to an already created page too.

Objects on a Master Page should have one of three modes. 
1. Object is locked when editing a page that uses this master. It can't even be selected.
2. Object is a media placeholder.
3. Object is freely editable on a page created with this master.

I might create a text box to hold the page number, and use the first mode to lock it in place. I might also have a text box for the main story content, and use the second.

Layout guides placed in a master would be shown on the page created with it. Of course, if the user goes back to edit the master and changes the guides, the updated guides would be shown when they went back to edit a page created with this master.