Enable FOP's PCL duplexing feature with DocBook XSL stylesheet customization?

[Warren Young <warren@etr-usa.com>]

I’ve just learned about a feature of FOP which allows you to enable duplex printing when generating PCL output from FO input:

    https://xmlgraphics.apache.org/fop/2.2/output.html#pcl-duplex-mode

Unfortunately, grepping the DBX FO XSL files in the version of the stylesheets packaged for my OS turns up no instances of “duplex”.

That leads me to wonder what the sanest way to insert this directive is:

1. sed the FO output.  Ewwww.

2. Find some XML tool that takes an XPath or similar and lets you insert XML elements at points matching the query.  Recommendations?  I’m hoping for something with the sensibility of jq for JSON, only for XML.

3. Maybe there is a trick with XSL customization files to insert arbitrary directives?

4. Prevail on those maintaining the standard DocBook stylesheets to add a way to turn this feature on in a standard way, so that in about a decade’s time when all my machines with outdated docbook-style-xsl RPMs are replaced, I’ll be able to turn it on with a one-line XSL directive. :)