Using ms: xpath functions inside XPathExpression

Question

I am trying to use Microsoft XPath Extension Functions (such as ms:string-compare http://msdn.microsoft.com/en-us/library/ms256114.aspx) inside an XPathExpression object.

These functions are extensions inside the MSXML library, and if I use them in an XslCompiledTransform (simply adding the "ms" namespace) they work like a charm:

                string xsl = @"<?xml version=""1.0"" encoding=""UTF-8""?>
<xsl:stylesheet version=""2.0"" xmlns:xsl=""http://www.w3.org/1999/XSL/Transform"" 
    xmlns:xs=""http://www.w3.org/2001/XMLSchema"" 
    xmlns:fn=""http://www.w3.org/2005/xpath-functions"" 
    xmlns:ms=""urn:schemas-microsoft-com:xslt"">

";

            XmlDocument xslDocument = new XmlDocument();
            xslDocument.LoadXml(xsl);

            XslCompiledTransform transform = new XslCompiledTransform();
            transform.Load(xslDocument);

Then I tried using them in an XPathExpression:

        XPathNavigator nav = document.DocumentElement.CreateNavigator();
        XPathExpression expr = nav.Compile("ms:string-compare(/Data/@timeout1, /Data/@timeout2)");

        XmlNamespaceManager manager = new XmlNamespaceManager(document.NameTable);
        manager.AddNamespace("ms", "urn:schemas-microsoft-com:xslt");
        expr.SetContext(manager);

        nav.Evaluate(expr);

But I get an exception "XsltContext is needed for this query because of an unknown function".

XsltContext is a specific XmlNamespaceManager, but I don't know if it's possible to instantiate it without an actual XslCompiledTransform (it's abstract) and use it as my expression context.

Is there any way to do this (or any other way to use ms: extensions inside an XPathExpression)?