The method I'm using relies on the magma tunnel to be at the top of the magma source. If that's the case, dig your magma tunnel up to one tile away from the magma source. Dig a corresponding tunnel above it, and then channel the next tile in the top tunnel to make a breach. I usually brick up the top tunnel an have magma safe drawbridges in the lower one.
However, the next time I need to breach magma somewhere in the middle (typically in a volcano embark), I'm going to try to grasp the bridge version (I've previously used the method above, with staircases to let breaching magma flow down. That usually works, but not always).
@NullForceOmega:
If I understand your setup correctly, I'd rather use a drawbridge than a door because of building destroyers (although I don't know of any magma dwelling building destroyers, so it might not matter).
With the jobs rewrite you don't even have to time the designation of the next engraver job, since you can just assign the breaching one a higher priority than the "get out of there!" one. Since a fortification carving requires smoothing first, you typically want to micro manage it anyway, though (it's a bit of a pity you cannot queue up a fortification carving on top of a smoothing designation. I work around that with a "dummy" smoothing beside the fortification to be smoothing so the bugger doesn't run away between the passes, but it obviously requires micro management to time the fortification carving, but several dummy smoothings (that are removed when the fortification one is designated) could help keeping the bugger in place).