The TaskClient is the API that you should be using for normal operations, and it's defined by the WS-HumanTask specification.
At this point, there are no operations to simply modify a Task randomly, as it's not defined in the specification. It shouldn't be too difficult to add, if you just look at TaskServiceSession.addTask, I guess we could just add a similar one for saveTask that would just save the task using the entity manager. If you might be interested in giving that a try, I can try and help and guide you through it?
Kris