I've been dabbling with the logging portion of JBPM for a while now and I'm currently adding a custom log class. (I need to know who started a new process instance, see my post in the user's forum).
I think a worthwile addition to the documentation would be a section ( more like a paragraph) on how to add custom log classes: by extending ProcessLog, creating the corresponding hbm.xml file, etc. (correct me if I'm wrong, please!)
I'm pretty sure I'm not the only one who knows almost nothing about hibernate and so I had to search a little to deduce what I needed to do to persist the new class "seemlessly" with the rest of JBPM. Thankfully, section 9.8 had my answers.
On a side note, I'm surprised to see there seems to be so little interest in the logging dept., as it is crucial to some application domains. Maybe they use their own solutions?