The caisson was specifically designed to be filled with water. Unfortunately, I've discovered that it is apparently impossible to obsidianize magma immediately above a magma flow. It just vaporizes that square of magma, which then refills quite quickly. The end result is that the water being piped in from the lowest cavern layer can't reach the breach to seal it off, because there's a square of magma in the way that is above a magma flow. For the time being, I'm allowing water to flow in freely, in the hopes that, by chance, it might seal the breach. There are levels of FPS I am prepared to accept.
It would have been easy enough to purposefully breach the HFS and let them die to a trap. I've done it several times before with other forts, though never in a manner as stylish as Aussie's (I prefer the long series of vertical switchbacks with a controlled cave-in at the top). However, I dug it out completely by accident, and now the demons are so spread out its impossible to defeat them all now.
A trio of Pterosaur Devils have made it to the surface somehow, and are currently in the process of tearing an entire goblin siege to pieces. I suspect they swam up the magma vent (it's the most direct route), but its also possible they made it through the mines. In any case, it's only a matter of time before one, or a few, or several demons find their way into the dome. For the dwarves, life continues, idyllic and serene now that they are content in the knowledge that they really won't be the last of their race, dying alone and forgotten in the deeps.