Using a custom field for the author link

We use guest bloggers on our site and need a way to manage the displayed author names and guest author links.

Following the instructions here:
http://www.wpbeginner.com/wp-tutorials/how-to-rewrite-guest-author-name-with-custom-fields-in-wordpress/

I am able to change the name of the author to display the guest blogger’s name.. However the author link still points to the registered user who posted the blog.

So now I’d like to do something similar with the author link. What I would like to do is for the author link to come from a URL that I will enter in a custom field. This allows the guest blogger name to link to their own profile page or even their own website.. Following a suggestion from this thread: http://wordpress.org/support/topic/how-to-handle-guest-authors-without-creating-profiles?replies=9, here’s the code I’m using..

add_filter( 'the_author_link', 'guest_author_link', 10, 3);
add_filter( 'get_the_author_link', 'guest_author_link', 10, 3);

function guest_author_link( $link, $author_id, $author_nicename ) {
  global $post;
  $url = get_post_meta( $post->ID, 'guest-url', true );
  if( $url )
      $link = $url;
  return $link;
}

This isn’t working and I could use some help..