I made a raising bridge that opened+closed 1 tick later than a retracting bridge directly overhead, both linked to a plate triggered upon entering the system. Not very useful. Upon retracting the bridge, dwarves go into free-fall state, but of course they have nowhere to fall because of the raising bridge 1 z-level under them. They don't move or drop their item until the retracting bridge closes-- then they cancel their job and drop their item. The item itself, once dropped, won't fall-- probably uses the same falling code as dwarves, maybe instead same code as water, takes a few ticks to fall, so can't fall in the 1 tick window allowed it.
You could take advantage of this technique to separate items from dwarves, but it's a pain. First, you've got to autodrop dwarves, which requires long bridges. Next, to get the goods someplace safely, you've probably got to use long rows of hanging floodgates (rather than a raising bridge underneath). You need separate entrance and exit actions, so one-way entrance: assuming initial state of bridge and floodgates closed, entrance triggers the bridge (open+close is fine), and exit triggers both the bridge and the floodgates underneath (again, open+close, to return to initial state). Probably want the exit blocking off entrance for a while, too-- hatch/retracting bridge combo will keep it closed during the necessary period (200 ticks). You might have extremely rare instances of plating of goods, when a dwarf is so slow he takes 100 ticks to move a tile-- basically, goods will be plated at the same rate that a plate-hatch undump lure tile needs ungunking, which I think is an acceptable rate of rot.
You'd still have the problem of dwarves delivering a lot of empty barrels for a booze stockpile, but now that I think about it, I bet you can get around this with some orientation tricks.