Just a question, so do you think you will add an option in future versions to put related posts in RSS feeds themselves? Or should I go ahead and do the following as you mentioned:
You can make this change by editing your wp-includes/feed.php. You can try to append related_posts()to the $content in the definition of the the_content_rss() function. I haven’t tried it myself so I can’t guarantee that it would work.
I only ask, because if you think you will make the change, one can wait until it is included instead of editing code and then “forgetting” to remove that code if you were to integrate it into the plug-in yourself. Cause I do think it is a great idea to include the related posts in the RSS feed, since I know of many people who do not “visit website anymore” but just use the RSS feed.