Currently it's not easy to dump the XML to stdout.
The only way is to trick the program using --dir=/dev --filename_format=stdout
But even then, it won't work if command is piped (blocking/non-blocking IO ?)
Would it be possible to have a -o|--output flag which, if value is "-", is considered as stdout.

Then we could do:
wp --quiet --post_type=foo --post_status=publish -o - | xsltproc -stringparam hostreplace https://preproduction make-preprod.xsl - | xz -9 | gpg -c > dump.xml.xz.gpg
See humanmade/WordPress-Importer#136 about the hope for adding | ssh preprod "wp wxr-importer import -" to the chain