Here (can't believe I'm writing this) is a module designed to create xhtml on the server side, to be used with the Community Starter Kit from Microsoft. It is written in C# and .Net framework. It is essentially a custom module component, which in itself is pretty small.
This little piece of .Net code takes an xpath (that you provide) and uses it to navigate through an xml file (that you specify), then taking the tags (that you specify) and producing a RSS feed from it. The RSS feed in question is compatible with the Microsoft Community Starter Kit RDF web box feature. Whether it's more generally acceptable remains to be seen. Bugs: it's probably a little tough to use...