As you said per page/post basis, this would work for each post
add_action('wp_head', 'add_link_in_head');
function add_link_in_head()
{
global $post;
if(!empty($post))
{
$alternate = get_post_meta($post->ID, 'alternate', true);
$hreflang = get_post_meta($post->ID, 'hreflang', true);
if(!empty($alternate) && !empty($hreflang))
{
?>
<link rel="alternate" href="<?php echo $alternate; ?>" hreflang="<?php echo $hreflang; ?>" />
<?php
}
}
}
If there is no $alternate and $hreflang then there would be no link.
