I must say that I don't like ExternalActivityBehaviour interface, it should be design to handle much more scenarioes, not just for the signal.
Can't agree more with you, but right now we must focus on bug fixes instead of enhancements.
I am very interesting in the EventActivityBehaviour in the current distribution of Activiti-5.0-alpha1. It is worth to refer.
We can revisit this point after releasing 4.4.