Per the devlog, it appears that we have the following at our disposal:
* Minecarts that can be filled with liquid ("The carts currently fill up with liquid when they are in a liquid square")
* Minecarts that derail at high speed & do damage ("The mass/speed will matter in the same way as it does with weapons, since it'll likely just call the same function.")
* Minecarts are made of wood & metal, so lead or even silver will be !!FUN!!. ("The heaviest fast minecarts will blast through many dwarves before they are slowed significantly.")
* Pressure plates that detect minecarts and how full they are.
* Powered rollers to accelerate carts, as well as stop zones.
* Newton's Cradle works (minecarts can transfer momentum)
* Bridges act as rails for minecarts
The two most important points is that rollers are powered (so they should be controllable with Dwarven logic). So what we need to do is have a setup like this:
(Loading Zone)===(Magma Channel)===(Long Holding Track)==(Logic-Powered Rollers)======== => (long, thin hallway where enemies eat hot lead from derailed carts)
Presumably, we can use rollers at the back of the holding track to bump a single cart into the zone in front, Newton's Cradle style, where it goes into the mass of rollers in the accelerator zone and gets shot directly into our enemies. If rollers can't do enough, we'll just have to build massive downward ramps to finish the job. Once we hook this to, say, pressure plates, we can have it so that a hot, flaming lead minecart zooms down the ramp, into the goblin siege whenever they step on the pressure plate. It may also be necessary to use raising bridges to keep carts from spilling out by mistake.
Just make sure you don't let your legendary axedwarves stand in that hallway by mistake!