Remco,
Try this little tool "IBM XSL Formatting Objects Composer", it does XML, PDF, and XSL and more.
Very, very good.
http://alphaworks.ibm.com/tech/xfc
Only problem is that you need Domino 6.0 or above, and you need to have a Windows platform (since I do not think that it is available on any other platform).
It is of course installed on the server, so the client does not need to have anything special to be able to convert to XML.
The other thing to bear in mind is that it is only for data, you cannot convert design to XML with this product (that unfortunately is a problem for me).
Good luck.