Procuding XML is actually not impossible. I defined an ADP that generates an RSS XML document. All you need to do is correctly set the content-type etc in the Tcl file so the headers end up correct. I used an index.vuh so that the filename was rss.xml.
I found this the easiest way to take the variables defined in a Tcl file and turn them into an XML document. See https://openacs.org/forums/message-view?message_id=267842
