styling nav links

I’m trying to do this on the pages navigation list on my wordpress site.

As it says, to make this work you need to wrap a span tag around the word.

But in WordPress, the nav list gets dynamically generated with this bit of PHP code.

<?php wp_list_pages('title_li='); ?>

..and so I can’t access the markup for the list itself to put the spans in.

Does anyone know a way of doing this?