I read the pages.dtd to find the action element and got it working with that. Here is how I did it:
1. Add a String property "selectedTab" to the backing EntityHome of the main page;
2. In each child page's Seam page config XML (child.page.xml), add an action element that calls "setSelectedTab('value')";
3. On the main page, using EL to value the selectedTab property of the rich:tabPanel tag.
I feel that introducing one more property in the backing bean may not be necessary (it also pollutes the EntityHome). I would imagine there be a way to pass parameters around the pages and use their values in EL, but I don't know how. I saw that in pages.dtd, there are also "in" and "out" elements, wonder if these can do exactly what I wanted, but couldn't find their description/usage on the reference doc. Any clarification/education?