I've watched drawbridges fling people a fair distance before(middle of 5x5 map to the edge with no obstacles between bridge and landing site), but it was in the direction the bridge lowered, not the raise direction. I'll agree that there's no reliability with them, though. The retracting bridge method would work great, it's like that combat drop tactic I hear about only you don't have to worry the user landing safely. The only downside would be the time and resources needed to build something like that on the surface. Not that much really but something I have to consider if I want to keep my workers alive.
So far you've all given me four ways to make this work. This is what I've interpreted from what you've given me so far.
1. Using DFhack to enable non-stone ammunition, fling corpses by catapult and reanimate them when they land(because projectiles cannot be interacted with mid-flight)
2. Use a minecart launch tube to fling corpses and other objects(possibly including the cart itself, depending on construction), and reanimate them when they land(because they would dismount the minecart if animated in transit). Designing the system so the launch tube ends at a cliff of any height would prevent the animated corpses from going the wrong way and causing trouble. NOT designing the system to prevent this would be the truly dwarven way of things.
3. Gamble with a bridge-a-pult and hope the zombies land where they need to and not in the middle of a party held at a statue garden. Corpses can be animated before or after they land. This method gains points with me for how random and potentially entertaining it could be.
4. Drop the undead from above with a retracting drawbridge. Corpses can be animated before and after they land, possibly both depending on how the system is built. Unless constructed underground, this system would require time, resources, and exposing workers to attack in order to be set up.
I don't see any reason I can't try all of these. These methods could all work any situation given enough effort, but they're easier to implement in different circumstances. For instance, the retracting bridge drop method would be a useful delivery mechanism for inside the fortress, in the caverns, or any area underneath the player's general territory. The way I envision the use of minecart shotguns would be ideal for defending a mountainside fortress from the outside, and could be built into the outer walls of any defense. All of these suggestions are good and would work fine if I try hard enough, but they'll shine brightest if I can find the right environment for them.