Multi-site menu attempt

global_menu.php

<?php
//the slot is the menu location string you registered for the global menu
$slot = "menu_name_here";

function global_multisite_nav_menu_filter( $content, $args ) {
	global $slot;
	
	$registered = get_registered_nav_menus();
	if( !isset( $registered[$slot] ) )
		return $content;
		
	if( !is_main_site() ) {
		$network_menu = get_site_option( 'ra_network_menu' );
		if( !empty( $network_menu ) )
			return $network_menu;
	} elseif( !get_option( 'ra_network_menu' ) ) {
		update_option( 'ra_network_menu', '1' );
		update_site_option( 'ra_network_menu', $content );
	}
			
	return $content;
}
add_filter( 'wp_nav_menu_objects', 'global_multisite_nav_menu_filter', 10, 2 );

function global_multisite_nav_menu_flush( $post_id, $post ) {
	if( is_main_site() && $post->post_type == 'nav_menu_item' )
		update_option( 'ra_network_menu', '' );
}
add_action( 'save_post', 'global_multisite_nav_menu_flush', 10, 2 );