Is it possible to create a standalone landing page that exists outside the main site? For example:
I can see that you can have pages not appear in the nav, but what I'm after is a page that has its own layout, nothing to do with the main site. Is there a simple way to achieve this, or am I better just uploading a static HTML page outside of SilverStripe?
This can easily be done. What you'd want is a different template applied to this page. Have you read the tutorials on the use of include files and the layout folder? http://doc.silverstripe.com/doku.php?id=tutorials
Also take a look at this page for using renderWith() - http://doc.silverstripe.com/doku.php?id=templates&s=renderwith#calling_templates_from_php_code