Great question!

This is one of the most under-documented areas of Seam.  If I'm wrong, please tell me (and google).  The best I've found is actually
this community contribution, but it too could use some elaboration.

Here's the JIRA link Link tracking this work. 

Also, check out the  Seam UI Refcard by Jacob Orshalick, which has a section on pages.xml.