The Jenkins process runs as the jenkins user, but when oXygen is invoked, it tries to read/write from
/root/.com.oxygenxml, and since that directory is owned by root, it fails. I've chowned it to jenkins:root (jenkins appears to have no group), and I'm trying again. I've also created /root/.java, since oXygen seems to need that, and made it readable by the jenkins user.
I also discovered, from the remaining errors in the Stylesheets task, that there is no symbolic link trail for the Java jar executable. This is probably the result of my installing the Sun JDK manually instead of from the repos. So I did this, following the analogy of the java symbolic link:
sudo ln -s /usr/local/java/jdk1.6.0_25/bin/jar jar sudo ln -s /etc/alternatives/jar jar
And now the Stylesheets task completes!
Now there's just the blasted Han Nom font issue.