How do I get the XML string out of the MvpXmlTransform?

The sample code provided is this:

using System;
using System.Xml.XPath;
using Mvp.Xml.Common.Xsl;

public class ExsltTest  
{
  public static void Main() 
  {                    
    MvpXslTransform xslt = new MvpXslTransform();              
    xslt.Load("foo.xsl");              
    xslt.Transform(new XmlInput("foo.xml"), new XmlOutput("result.html"));          
  }      
}

But instead of outputting the XML to a file I want to do some in-memory string processing on it. But I haven't found a way of getting to it.
n.b. I want to handle it as a string, not as an XML document.

Is there some XmlOutput class that wraps a string variable? Or is there some XmlReader class that does the same?