//attaches in-reply-to classes to the first link in the post
function the_content_in_reply_to_filter($content) {
  return preg_replace('`\<[aA] (.*?)>(.*?)\@(.*?)</[aA]>`','<a class="u-in-reply-to" rel="in-reply-to" $1>$2@$3</a>',$content,1);
}

add_filter( 'the_content', 'the_content_in_reply_to_filter' );