The undump thread had something that might work, no necessary. I believe you can put stockpiles on floor hatches, so you could just have the hauler trigger it to drop whenever they leave. You might need some sort of one way entrance thing so they don't cancel the job when it opens in front of them, though. Not sure if that would actually be a problem, so long as the pressure plate isn't directly adjacent to the stockpile, and there's only one stockpile tile.
It's possible though the big challenge is that, as far as I know, everything which can move an item, can also move a dwarf and items and dwarves being moved by external forces follow the same rules. This means that the item shunt/fling/drop needs to only happen when dwarves are excluded. Essentially you need a "dwarvern computer" which closes entry to the stockpile, waits for dwarves to clear from it, then flushes the items down a z-level or perhaps through fortifications.
Well actually is one difference between dwarves and items, in that dwarves can also move under their own power - provided they aren't stunned (and a dwarf who falls a z-level IS stunned!). So it would be potentially possible to make a "conveyor belt" of sorts which pushes items along horizontally while allowing dwarves to get off, the problem however is that dwarves will tend to try and grab the items which are being pushed along - if they aren't on a stockpile tile - or if they are on a stockpile tile, they'll go to that stockpile tile to put items on it.
I think it is possible using a combination of pumps, staircases, pressure plates, hatches and restricted traffic zones - to make a system that automatically filters out items from dwarves and drops the items to a lower z-level without ever restricting dwarf access. While this would be dwarvern engineering at it's finest, it would probably be less effort to just use a dump zone every so often, or just use a burrow to "mind control" dwarves into their quarantine areas.
(but of course, now that I've conceived of this contraption, I just have to try and build it...)