"Dave" asked for opinions on his approach to supporting non-RSS elements in RSS feed.  His approach is to simply say "it's okay for an RSS feed to include elements not defined in the spec."  It sounds like a commonsense solution although I would also add a line describing how elements not defined in the specs should be handled.  They should obviously be ignored, but there could be elements that contain RSS elements so one must say whether to ignore contents of elements not defined in the spec.  My recommendation is: ignore unknown elements and their contents.