Back to Seaside, it also provides an API to generate RSS. Indeed, that is how this blog's feed is generated:

| doc ctx root rss |
String streamContents: [ :stream |
    doc := WAXmlDocument new initializeWithStream: stream codec: nil.
    ctx := WARenderContext new document: doc.
    root := RRRssRoot new openOn: doc.
    rss title: self siteTitle.
    rss description: self siteSlogan.
    self blog do: [ :ea |
        rss item: [
            rss title: ea title.
            rss author: self siteAuthor.
            rss link: self baseUrl, ea blogUrlPath.
            rss guid: self baseUrl, ea blogUrlPath.
            rss publicationDate: ea timestamp printHttpFormat.
            rss description: ea outputHtml ]].
    root closeOn: doc ]