Hi Cendric Hurst!
The start message event is supported in jbpm. In the jbpm-examle project (http://sourceforge.net/projects/jbpm/files/jBPM%205/jbpm-5.3.0.Final/jbpm-5.3.0.Final-examples.zip/download) are many examples of bpmn processes with junit-tests. There is one example with a start message event: testMessageStart() with BPMN2-MessageStart.bpmn2
In this example the process has the start message event: HelloMessage
The process starts by calling the signal: Message-HelloMessage
public void testMessageStart() throws Exception {
KnowledgeBase kbase = createKnowledgeBase("BPMN2-MessageStart.bpmn2");
StatefulKnowledgeSession ksession = createKnowledgeSession(kbase);
ksession.signalEvent("Message-HelloMessage", "NewValue");
}