Anton, it almost sounds like Derek is asking for thread-local-storage for the pool threads.
Derek, tasks in TBB are not persistent; they are created, queued (often) and executed. Though some may be recycled through continuation tasks, there are not a fixed set of tasks associated with the pipeline filters that might serve as accumulators. Usually such "reduction" operations are handled in TBB via parallel_reduce.
However, the threads comprising the workers that process the tasks in TBB are persistent, and if the pipeline interface is a requirement for your data input, I could imagine an interface in which a filter has thread local storage, available in the current OSS distribution as enumerable_thread_specific, where the filters did accumulation of data in such a thread private area. Of course, you'd need to ensure the private accumulators are initialized before the pipeline is executed and they would need to be harvested once the pipeline was compete. I think you could probably come up with a solution via this approach, though it would be much simpler I suspect if you can just use parallel_reduce.