There is, but it's rather complicated.
You need to use a series of Sketchup.send_action("pageNext:") commands until the Sketchup.pages.selected_page is the one you want to have active.
EDIT:
It appears there IS a pages.selected_page= method, that is undocumented. The method makes it UNcomplicated, but the undocumentedness re-complicates things