I would have at least an idea how to do this similar to your first idea (e.g. using pages.php as a wrapper to load other theme pages).
That could automatically load a script page with for example the name `pages-<titlelink of the page to use it>.php` if it exists. But I see that a selector might be more convenient for reusing layouts, I think that might be possible as a plugin (but will require the theme to be setup correctly).
If you like sent an e-mail.