Here is some code for SUnet for sending XML ouput.  Send-xml/suspend and send-xml/finish do what their twin brothers send-html/suspend and send-html/finish do, but set the Content-Type header of the HTTP response to "text/xml". The rules for transforming the SXML tree additionally support XML  processing instructions. Something like
  '(*PI* xml "version=\"1.0\"")
becomes
  <?xml version="1.0">
in the output. A second example:
  (*PI* xml-stylesheet "href=\"foo.xsl\" type=\"text/xsl\"")
becomes
  <?xml-stylesheet href="foo.xsl" type="text/xsl"?>

Here is the code:

  (define processing-instruction-rule
    `(*PI* *preorder*
  	 . ,(lambda (tag . elems)
  	      `(,(string-append "<?" (symbol->string (car elems)) " ")
  		,@(cdr elems)
  		"?>"))))
  
  (define xml-rules
    (cons processing-instruction-rule default-rules))
  
  (define (make-xml-reponse xml-string)
    (make-surflet-response
     (status-code ok)
     "text/xml"
     '(("Cache-Control" . "no-cache"))
     xml-string))
  
  (define (send-xml/suspend xml-tree-maker)
    (send/suspend
     (lambda (k-url)
       (make-xml-reponse
        (sxml->string (xml-tree-maker k-url)
  		    xml-rules)))))
  
  (define (send-xml/finish xml-tree)
    (send
     (make-xml-reponse
      (sxml->string xml-tree xml-rules))))

And here is a module definition for this code:

  (define-structure send-xml
      (export send-xml/finish
  	    send-xml/suspend)
    (open scheme
  	surflets/sxml
  	surflets/my-sxml
  	surflet-handler/primitives
  	surflet-handler/responses)
    (files send-xml))

-Eric