Hi Alexei,
I think it is some different from Task and Execution in the jPDL. Task is more likely a notification service to Human being. The Execution is the actually a part of Graph Oriented Language. If you use Discriminator Pattern, you may find it is difficult to implement it, but in our country, this scenario is very commonly. People want to have a most flexible human task service, so they want to decision who and could receive the task, you have to decide to create how much task may be created in the runtime. The different time you execute the process, the difference tasks may be created and assigned to different person.
It may sounds strange, but it is the real user requirement. I hope jBPM could support more powerful feature like this in future.