There's an easy way around this. Just ask the site owner to post it as their main author name, and leave the "guest post" wording off the article. There's no way in hell Google can determine a piece of content is a guest post unless there's some verbiage on the actual page saying "This is a guest post from John Doe". Site owners need to remove the big menu links that say "Write For Us", cause they're gonna get wacked by Google even if they've never accepted a guest post.