A proof-of-concept attempt to cast an entire fortress out of obsidian.

Lava source is an active volcano lava tube extending several z levels above the surface.

Water source is a carefully controlled river flowing into a dwarf-made lake.

Pics at imgur: http://imgur.com/a/dHJh2

The order of operations (using the Legend at imgur) is as follows:

At the time of save, the system is prepared to drop water upon the casting surface, pull level #4 to drop several thousand units of water upon the unsuspecting lava pool below.
Then close lever #4 again to close the retaining bridges in the casting reservoir.

Pull lever #2 to start up the pumpstack, and lever #3 to open the casting reservoir above.

WARNING: this will fill the above reservoir VERY QUICKLY! Do not let the casting reservoir above fill more than 1/4 of the way under pump pressure (i.e. when 1/4 of the casting reservoir is filled with water, shut off #3 to close the dam, then #2 to stop the pump stack) the water will then settle to about 2 or 3 deep across the whole casting reservoir. (it takes time for the lake below to refill to usable depth.

Wait until the new casting surface is full of lava at least 2 deep. (I recommend waiting until 5 or 6 deep to allow the storage lake to refill sufficiently)

And then start back over at the top of this process by pulling lever #4