How to use wp_nav_menu to create a select menu dropdown?

Question

I'm using the following inside the wp_nav_menu function to create a select dropdown menu where each menu item is an option in the select dropdown...

'items_wrap' => '<select>%3$s</select>'
'before'     => '<option value="">'
'after'      => '</option>'

How do I add the link value in the 'before' declaration? Is there a better way to go about this? I know about wp_dropdown_pages, but that doesn't work because I want users to be able to control the menu from the "Menus" page.