dynamic tiles configuration in struts1.2.x?

Folks, I have an existing project which makes heavy use of tiles definitions for pages. now there's a requirement to change the pages shown based on the user. Any ideas on how to do this would be greatly appreciated. I would of course prefer not to create a static tiles definition for each page

basic question about s:iterator

Sorry if this is too basic, but i seem to be missing something. The iterator tag documentation says it iterates over the value on top of the stack. I don't understand what this stack is and what is at the top of the stack. Can i use the iterator tag to iterate over an object stored as a session