Hi Michael,
the task lifecycle is indeed a great idea. It has not yet been used in jBPM 4.x. The ideas shown and implemetend there are for sure the way to go for the future . We have not taken them into account now because at the moment we are focusing on fixing bugs and not implementing new features. In case you have read the request for comments on jBPM 5 you will have noticed that they are taking WS-HumanTask into account which also uses a task lifecycle to deal with tasks.
Cheers
Sebastian