How can I access a post’s feature image outside the loop?

On a single post page the $post variable should work for you. It is set well before the page header prints. You will need get_post_thumbnail_id and wp_get_attachment_image_src

 if (is_single()) {
    $thumbid = get_post_thumbnail_id($post->ID);
    $imgsrc = wp_get_attachment_image_src($thumbid);
    $src = $imgsrc[0];
    //   wp_die(var_dump($src)); // debugging/confirmation
}

I don’t know how your theme is written or how that meta tag is inserted so I can’t say exactly how to insert the image source into that tag, but the above will get you the URL.

$src will be NULL if there is no featured image so use isset or empty to check the variable before using it.