I don't know how the eclipse plugin work, but I know for sure that the Start Button is calling the TaskClient.start() method.
At this point I must ask, if you really need to use the eclipse plugin? you will usually will need to build your own app for your end users right?
When you build your own app you will see that you will have those hook points.
If you want to see an example about this you can take a look at here:
https://github.com/Salaboy/jBPM5-Developer-Guide/tree/master/chapter_07/jBPM5-HumanTaskUI-Swing
That project is a simple Human Task client built using Swing, it also has a start button, and there you can go and add your logic. But once again, take this as an example, because you will probably need to build your own app for that kind of things.
Cheers