An update: It was possible to evaluate XPath queries on virtual documents only at the time of writing of this article. We have moved forward in the meantime. The latest CVS version of Sablotron can run XSLT on external documents as well. This feature should be included in the very next release (0.9x, where x>0).
Shortly from the answer: version capable to process external documents is on CVS, but not yet on the main trunk, check-out instructions are included in the message, as well as some documentation updates that reflect the latest changes.