While I think this particular problems can be addressed as above, I
agree that it's often helpful to be able to get hold of the URI for
the source document, and even the stylesheet, for example to add
comments like:

<!-- transformed from source.xml by my favourite XSLT processor -->

Perhaps a base-uri() function or extending the system-property()
function:

  system-property('xsl:source-uri')
  system-property('xsl:stylesheet-uri')

--
Oleg Tkachenko
Multiconn International, Israel