The challenge with trying to use bridges as a conveyor is that the direction which objects are thrown is completely random. Objects which are on a retracting bridge when it retracts, or which are resting on the space the bridge extends into when it extends, will become airborne and will move until they either hit a solid object or have moved some distance away. Objects can be thrown up to 3 tiles away X,Y, and Z, in any direction. The direction they travel seems to have nothing whatsoever to do with the shape or size of the bridge. You can use walls or buildings around the bridges to stop them from moving in directions you don't want them to, but you can't really channel them as such. Raising drawbridges throw objects randomly up to 11 tiles away, except for objects which are on the side of the bridge that turns into a wall, which are atomsmashed. Lowering drawbridges of course atomsmash everything in the area they lower into.
In my 40d fortress Pagedslipped I built a conveyor belt out of retracting bridges that looked as follows from the side:
................................
.......B.......B.......B.......B
......BW......BW......BW......BW
.....BWW.....BWW.....BWW.....BWW
....BWWW....BWWW....BWWW....BWWW
BBBBWWWWBBBBWWWWBBBBWWWWBBBBWWWW
-> objects move left to right ->
where every . is open space, W is a wall, and B is a retracting bridge on solid floor attached to a water-powered fast repeater. Every 100 ticks every bridge in the device would retract or extend. Invaders would fall onto the array from a great height, splattering body parts and invader gear on the bridges. Each time the bridges extended or retracted all the objects would fly around randomly. Objects in the array would mostly bounce around in the flat parts, but eventually after enough random movement some would walk up the slope to the right of each section, and fall over the peaks into the next flat section. Because of the way the array was built, objects could not move backwards towards the left, and gradually would all be moved towards the right, to a safe collection area inside the fortress.
This conveyor belt mechanism worked. It also took forever to build, was very slow (objects could bounce around inside it for years before reaching the end) and causes a massive FPS hit every 100 cycles from all the objects flying around.
There is a trick I considered but haven't tried yet that could be used to make it more efficient. It turns out that it is actually possible to build doors floating in empty space. Well, you can't actually build a door over empty space, but what you can do is construct a wall, then build a door on top of that wall, and then remove the wall. This will leave the door floating in empty space. At least, it did in 40d. I haven't tried it in DF2010 yet.
So in theory you could build a mechanism as such:
WWWDB
WWDBW
WDBWW
DBWWW
where each D is a door floating over an open space, and each B is a retracting bridge. If you open and close the doors and trigger the bridges in the right sequence, you'd get a system that pumps objects up the steps and doesn't let them fall backwards. Objects can't get stuck in the doors, because each door is built over open space so any objects that would be in the door just fall onto the bridge below. The water logic to run the sequence automatically will be complicated. I'm not sure if there's any actual benefit from the device, either - I suspect that once you've got it running it would have been less work just to use an army of haulers - but the same can be said for many things I install in my fortresses.