It's not very difficult to update the selection.
Open a window in your perspective
Select all of the projects in the sidebar
Unfocus the view with the Show All button in the toolbar, or View->Show All Projects
Command-click on projects in the sidebar to toggle them in or out of the selection without disturbing the existing selection
Focus the view again with the Focus button in the toolbar, or View->Focus on <selected projects>
Perspectives->Take Snapshot of <perspective name>
You just need to avoid clearing the selection or you have to start all over again.