TclXSLT http:// ... is the XSLT accessory to the TclXML project. It wraps the GNOME libxslt [L1 ]. In a separate but related project, Steve Ball is wrapping libxml with a TclDOM-compatible API.
Steve Ball has a minimal example of how he calls back to Tcl to handle a common need for an external reference with TclXSLT [L2 ].