If you want to go directly to the fullscreen version of a gallery within a WordPress post, you can add #expanded to the URL of the post containing the gallery, e.g. http://www.example.com/wordpress/?p=123#expanded
You would need to modify WordPress source files to achieve this though.
You would need to find where WordPress generates the links to posts from the main overview page (and also the links to previous and next posts from individual post pages), add a check to see if the post contains a Juicebox shortcode and, if it does, add #expanded to the link URL.