here is how i would do that:

    add_action('save_post','update_all_meta');
    
    function update_all_meta($post_id){
    	// verify if this is an auto save routine. 
    	// If it is our form has not been submitted, so we dont want to do anything
    	if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) 
    		return;
    	//check for post type
    	if ( $post->post_type != "YOUR_CUSTOM_TYPE" ) 
    		return;
    	global $post;
    	$tmp_post = $post;
    	$args = array( 
    		'posts_per_page' => -1,
    		'post_type' => 'YOUR_CUSTOM_TYPE',
    		'post__not_in' => (array)$post->ID //skip current post
    		);
    
    	$myposts = get_posts( $args );
    	//loop over all custom posts and update the meta
    	foreach( $myposts as $post ) {
    		setup_postdata($post); 
    		update_post_meta($post->ID,'meta_key',$meta_value);
    	}
    	$post = $tmp_post;
    }

but if the stored data is the same maybe use options api so you only update it one and you can use it for all posts.