Excuse me, I was incorrect-- eventually, a dwarf working in the 0-tick magma tile began bleeding, and losing all of his fat. (I tested with temp both on and off, and may have missed some effects if I didn't do things exactly the same under both conditions.) If people still are interested in dwarven liposuction, I believe this represents a relatively safe and easy way to burn the fat off of dwarven bodies. (Note that the affected dwarf's clothes had already rotted off, so you might want to be a little careful regarding worn items). Further research ideas:
a) Will 0-tick water extinguish fires?
b) Can you bring water to the top of a pump stack in freezing conditions by building top-to-bottom? (Such a system should be a dead-simple hail generator.)
c) Can you pump magma through the same space that you pump water, at the same time or rather on alternating ticks, without creating obsidian? (Such a system might be useful for freeze-proof pump stacks.)
I've designed, but not yet built, a device that I think is novel, dwarfy, and effective, based on build-order principles: the magma curtain. Edit: But it doesn't work, so I've put all of my discussion of it inside spoiler tags.
##*### gear to power
# %%7# left-to-right pump
#^%%7# critter-triggered plate, right-to-left pump
####7# full (very important!), sealed reservoir
######
cross-section of hallway containing magma curtain
build order: lower pump, gear, upper pump, pressure plate (linked to gear)
Basically, when you turn it on, it fills the affected tile with magma, but not just any magma-- magma that doesn't flow. It's a wall of fire. You could put it in open space, or build a usable well over it (with a water version) if you wanted. You could fill one level with water and the level above it with magma, no floor between, and the two would never mix.
I believe that such a device will be fluid-conserving, have zero latency (for either turning on or off, although I believe a 0-latency both-ways version is also possible), and block flyers (unlike hatch-over-channel devices). I also believe a version capable of creating a three-lane magma curtain is possible, by pumping through a u-bend, up through a grate. I need to do some testing to see how building destroyers deal with grates covered with magma (will they path or not?) and the impassable tile of screw pumps.
A water version should work as a contamination-free dwarven bathtub-- a dwarven pressure-washer. With dry land a single tile in every direction, I doubt that drowning is a risk, but a little swimming skill might be an extra bonus. I'm building a water version first, because I want to see how it interrupts dwarves-- a better quantum stockpile might be possible.
It's going to take me a while to exhaust the possibilities for zero-latency path manipulation, without requiring a floor tile. For instance, a goblin could be constrained to a single drop hatch, by closing in on him with walls of water.
Obviously, I'm getting a little ahead of myself.
rather than just the outlet space, what about the middle layers of the stack?
I tested a two-pump stack. In my stack, magma spent a fraction of a tick in the tile I was examining-- it was neither present at the beginning of the tick nor the end of the tick. I expect the results from the two-pump stack to apply to a stack of any height.