This plugin by David Engel changes a post’s link in the RSS feed when you add a custom field called ‘url’ in a post. If a custom field called ‘url’ is not present on a post, the link will be the post’s permalink.
Helpful when using WordPress to distribute RSS feeds that link to external pages.
Adapted from Dan Philibin’s tutorial at http://wpcandy.com/articles/tutorials/change-a-posts-rss-permalink-using-custom-fields.html
Plugin homepage: http://www.engeljournal.com/tools/word-press-plugin-replace-rss-feed-link/
