FOP stands for Formatting Objects Processor. This is the name of Apache's [L1 ] xsl-fo processor for XML. It basically allows conversion of XML documents into binary formats, for instance PDF. Currently only a Java version is available, which could be used with TclBlend or Jacl, or directly exec'd from Tcl.
I (NEM) sometime dream about writing an xsl-fo processor for tdom, but it would be a huge project (read the xsl-fo specs from the w3c for an idea of the complexity involved).