Filter for Admin Nav Menus Drop Down

I believe the filter you want is wp_get_nav_menus:

function wp_get_nav_menus( $args = array() ) {
    $defaults = array( 'hide_empty' => false, 'orderby' => 'name' );
    $args = wp_parse_args( $args, $defaults );

     /**
     * Filter the navigation menu objects being returned.
     *
     * @since 3.0.0
     *
     * @see get_terms()
     *
     * @param array $menus An array of menu objects.
     * @param array $args  An array of arguments used to retrieve menu objects.
     */
    return apply_filters( 'wp_get_nav_menus', get_terms( 'nav_menu',  $args), $args );
}

The wp_get_nav_menus() function is used on line 383 of wp-admin-nav-menus.php to set the $nav_menus variable which is used throughout the file in a number of places, so be very careful what you do.