Ran into the issue I've described on http://community.jboss.org/thread/148981 and gave a few attempts to answer it, but none of them seem to be acceptable. I ended up by bringing ExecutionImpl.java into my project and doing the following:

•in #initializeProcessInstance method, removed a call to composeIds()
•in #save(), added a cal to composeIds() before dbSession.save()

The downside is - the downside to it is composeId() being called twice for createExecution().