It works in steel making, you can use digging from beneath a bridge or digging + another dig 1 job elsewhere once the miner has started digging (+ safety water trench right behind miner if they still catch on fire), and as to having magma sources safely on hand, nothing can get up through floor grate or move through a statue without being thrown (true, magma sea has building destroyers, but if you build statue right before a downward ramp they'll be unable to topple it).
(Or if you're content with natural flow, can just use pond zone/bridges to seal up afterwards. Note that natural flow can take months to cross 200 tiles 4+/7 deep.)
Im generally better with pictures than words, but this sounds fairly straight forward, also sounds much less hard than the insane thing I was planning, I was thinking pressurized water, send in a miner to mine the last square into the tunnel (this might kill the miner obviously, I could place a puddle in a small escape hall such that the magma would hit that water and become an obsidian plug, retrieve the miner via stairs or ramp), open the water up just as they finish so it shoots down a tunnel and hits the magma, with the goal of making a obsidian cap inside the magma tube, such that I could mine out a square and stick a pump over the hole to the magma level below that one to access the magma via pump, remove the water system, build a short path for the lava to get pumped through that ends in a bunch of magma workshops, I have a magma pipe (calls itself a volcano, but its just a big circular hole full of lava in the middle of a flat plane), that goes from the surface to the sea, so I could build workshops on every needed level in a cluster around the pipe. I also just noticed you actually posted the worldgen im using I think, sleepervolcanoes or something, I modified it slightly to try and get more necromancer towers and removed the aquifer, one clusterfuck at a time, ill worry about learning to move ponds around and tap magma first, then ill worry about aquifers.
You have a weapon your forgetting. The king of battle, artillery. In my tests ballistae have a steady 200 tile range, and boast the ability of going thru multiple opponents.(and trees,dwarfs,animals) drawback : same z level, no up/down firing.
Train siege operators on the 230 tile range catapult, boasts no hit dwarfs, rids fort of excess rocks. Same z firing. Fairly weak, single opponent hits.
May take some work to funnel the bad guys, but once funneled the ballistae will mow them down. At ranges over 20 tiles the operators won't even see the enemy die.
Goatmaan
I actually have access to blood thorn wood, which supposedly makes for good ballista bolts, if I built a bolt catcher into my hallway to save ammo and those metal arrowhead things, would goblins and such just jump down the hole to the lower level, or will they just try to run down a long weaving hallway getting torn apart by ballista shots, and can you stagger catapults like you can ballista, such that I could get a line of three ballista, backed by 3 catapults, so that anything in my hallway is getting machine gunned by giant logs and boulders by 6 dwarves behind a wall of fortifications 20 or so deep so that invaders cant get close enough to scare them?
I like the idea of using ballistae but it seems to me that they only work well if you design your fort around them from the outset (in order to have an entrance with the necessary space for a shooting gallery) and that's not a compromise I'm prepared to make.
If only you could put them on towers and have them turn and fire down at enemies.
I have plenty of space, and whole top layer of my fort is useless clay, ive got deeper soil layers for farming, but ill place a clay workshop, thats about the only use for it [we're dwarves not hobbits, we live in stone, not mud, (why no halfling species I wonder) besides, I hate building much just below surface, have to constantly patch the holes in the roof as I cut down trees.] I too wish they could fire up or down levels though, but at that point you could cover a whole map with a few towers that could kill most anything by throwing trees and boulders at them all day long.