As far as I know, this is currently not possible through the admin interface. Manually surfing to content/view/full/1 will show the full view of the content tree's root node. Maybe if you let your browser show hidden input fields (with a bookmarklet like http://www.squarefree.com/bookmarklets/forms.html#show_hiddens ) and you manually change the node and object values then you might accomplish it too.
You can also make a script to do this. Take update/common/scripts/updatetoplevel.php as an example.
