There is a way to do this but it takes some coding.

First use UserControls rather than pages.  A usercontrol is generally a page
without the @page directive which is replaced with a @Control directive.

You can test all the UserControls as pages until they work if you want and
then replace the @page with @control and all should be good.

Now when you know the type of page/control you need in the content area
simply load the control and use it as if if were a page.  Now in reality
this approach does not need to use master pages.  You simply define a page
(which is your master) and a placeholder to hold the usercontrol (like a
content holder).

You have to remember to load the control in the page_init phase of the
lifecyle.  You can keep the type of control in session.

Hope this helps
Lloyd Sheen