Hi Alejandro,
Happy to see this issue finally was resolved. And we didn't use additional interface in the the jbpm-api.
I must say that I don't like ExternalActivityBehaviour interface, it should be design to handle much more scenarioes, not just for the signal. Since Tom and Joram just recently release their new BPMN engine - activiti. I am very interesting in the EventActivityBehaviour in the current distribution of Activiti-5.0-alpha1. It is worth to refer.