Feed Layout is designed to modify the content of posts when added to the blog feed. Feed Layout helps on adding headers, footers, copyright notices and so on.
In the control panel of Feed Layout you’ll find three textareas where put code (even PHP) which will be injected in every post content when elaborated for feeds.
WordPress feeds the contents in two flavours (settable from the settings): the excerpt or the full content. From version 2.5 (if I remember well) the “full content” is really the full content where, in previous versions, the full content was only the first part of the post, before the “more break”.
Feed Layout has a flag to force the second behaviour, breaking post content on the more break where extracted for feeds.
Available on WordPress plugin repository.