So, after searching the forums, I couldn't find a perfect method to tap magma that was 100% safe. The closest I found was the floodgate/door diagonal tap, which usually works...
Well, after my last fort falling to my sacrificial miner, who became a murderous ghost whose name never appeared on a slab, I decided to conduct some !!SCIENCE!! Here's what I came up with:
Embarked on a volcano with aquifer. Sadly, no surface water. HOT biome.
The classic understanding is: magma has no pressure.
I counter this with: magma DOES have pressure, but it's far thicker, and, therefore, doesn't behave like water.
I postulated this after watching a miner almost escape with the floodgate/door method, but burn before the door could close. I decided that it wasn't the door closing that gave the miner the chance, it was the diagonal acting as a pressure relief valve. But it wasn't enough.
So, to test this, I tapped the volcano from 2 diagonals, one from the tunnel to be filled, and then one further into the volcano. The result? 24 taps, no miner deaths. I was getting 1 in 5, at best, with the floodgate/door.
For the fun of it, I then breached an aquifer above the magma level, essentially flooring it off with cast obsidian. Several cave-ins, discovering the magma sea, and an adamantine pillar later, the volcano was half covered with floor, 2/7 deep in water... And lungfish came.
I'm currently breaching the aquifer further, as mentioned before, hot biome, it's summer and the water is evaporating after about 5 tiles of spread. But hey, soon I'll have a surface lake I can fish from