I am glad to see someone other than me performing and documenting actual experiments, especially in an area I haven't seen studied much. Very nice work, Sphalerite and winner.
I wonder if the rocks would travel farther away from the bridge if they weren't stopped by hitting the floor. It would be informative to repeat these experiments with a bridge built over a pit with another bridge built on each z-level to change its depth.
Also, those results with the airborne rocks remind me of some informal observations I made when single-stepping through the results of dropping invaders down a 10-level pit. If I recall correctly, the body parts ejected from splattering corpses went up to 3 z-levels high and travelled about 3 squares away. If you built a variable-depth pit, you could also use it to measure the injuries caused by falls of different heights, and the rules for how far a corpse has to fall in order to splatter and how the body parts fly when it does. People are always asking how many levels to dig to kill invaders, or reliably cripple hammerers without killing them. You'd just need a lot of kittens.
I hypothesize that there is one common "thrown objects" algorithm, and we'll see similar results for objects thrown by bridges, long falls, body parts cut off during combat, and creatures knocked into obstacles by hammers.