Future versions of XPathDocument may offer this functionality out of the box.
Important: this class requires unrestricted member access reflection permissions in order to run. If the Mvp.Xml assembly is installed in the GAC, it will run without problems. The Mvp.Xml allows partially trusted callers, so only this assembly needs to be GAC'ed.
Alternatively, you can configure .NET security policy to allow the appropriate permission (see ReflectionPermission, PermissionState.Unrestricted and ReflectionPermissionFlag.MemberAccess).