I suppose I could try and drain it using the map edge fortification bug(?) but that'll cause massive amounts of lag and thinking about my overly complicated cistern assembly, the fact that it's the magma sea would cause endless 'a section of the cavern has collapsed' spam.
The blue stuff vein gets wider as it goes down. dropping water around the vein should work for the first layer, the layers down should be managably large to dig out a little room within them.
I never thought about just running a bucket-brigade from a platform above it. Would that work?
Buckets don't drop enough water to form obsidian.