Resolved by creating standalone/configuration/session.template file and modifying the workItemHandlers group as follows:
workItemHandlers = [ "Human Task" : .....,
"Service Task" : ....,
"Twitter" : "new org.jbpm.process.workItem.twitter.TwitterHandler()"]
NB The documentation at http://people.redhat.com/kverlaen/repository/Twitter/ is slightly misleading. The that Twitter class is actually TwitterHandler not TwitterWorkItemHandler.