Maybe a screw pump powered by a windmill? Magma generally isn't pressurized unless it's forced, even if the flow isn't diagonal, and the location makes it one where the lifespan of a pump operator would be pretty brief.So for no reason at all, I decided to load up the save and test out the Kiwi Cumblaster.
The results? Frankly, I'm quite disappointed. Here are the results after about 4 months of open flow.
As you can see, it's hardly a Project Fuck The World-esque flooding. I thought that maybe it was because of the diagonal cut in the volanco's rim, but even after digging a channel into the rock blocking the way, it was still painfully slow. Perhaps the next guy building a wall directing the flow into our fort's courtyard could help?