This seemed to work.
package com.jbpm.test;
public class MyTransactionManagerLookup extends JNDITransactionManagerLookup {
protected String getName() {
return "java:jboss/TransactionManager";
}
public String getUserTransactionName() {
return "UserTransaction";
}
}
and modifying persistence.xml to
<property name="hibernate.transaction.manager_lookup_class" value="com.jbpm.test.MyTransactionManagerLookup" />