Hook or callback when wp_nav_menu() has completed

I am modifying the $output of wp_nav_menu() using a custom walker, and I need to run a function after the final closing /ul of the entire menu. I can only see the following methods available:

start_lvl()    // Runs when a child menu is started
end_lvl()      // Runs when a child menu is completed
start_el()     // Runs when a menu item is started
end_el()       // Runs when a menu item is completed

I can't find anything that runs when the top level menu is completed. Is this possible?