XMLHttpRequest and multipart responses

Does the XMLHttpRequest object support multipart/mixed response
content?

I am working on the FirePHP project (http://www.firephp.org/) which
sends multipart responses in order to embed debug information into the
response without affecting how Firefox renders the page.

A sample response body for a text/xml file looks like this:

--dc4b00ada3c698a5267b44d58caca336
Content-type: text/xml

<doc>
  <item name="test">TestValue</item>
</doc>

--dc4b00ada3c698a5267b44d58caca336
Content-type: text/firephp

DEBUG INFO

--dc4b00ada3c698a5267b44d58caca336--

When the XMLHttpRequest returns the responseText attribute contains
the entire response body and not just the text/xml section and the
responseXML attribute is null.

I presume this is the intended and proper result?
Is there any way to set an additional header to make the object just
read the text/xml section and load the XML document properly?

The other option would be to intercept and parse the response to
return the text/xml section only to the XMLHttpRequest object. Is this
possible?

You can see a demo here: http://www.firephp.org/Demo/Tests.htm
NOTE: You must have the Firebug and FirePHP extensions installed.

Any other suggestions or solutions?

Thanks
Christoph