You do not need to get to a context, there are two signal methods in StatefulKnowledgeSession.
One method passes event to all processes run by this session, another one passes event to a specific instance given its id:
ksession.signalEvent(signalName, data, instance.getWorkflowInstanceId());
Thanks Victor, I didn't get that from the user guide. This seems easier than what we used to have to do. I'll give it a try first thing tomorrow.