So, inspired by the great example of Boatmurdered (yeah friggin' right) I decided to try the whole lava-floods-world thing myself. It required a bit of cheating, unfortunately, and an active volcano on the map, and some trial-and-error-and-cheat-to-restore digging.
Unfortunately, for some reason the lava seems to have busted up my steel floodgates, or I maybe forgot to connect the levers correctly, or something else has gone wrong, and I can't turn it off. I've got a pretty good self-sufficent thing going with a population of about 75, but every siege, immigration, caravan and animal that enters the map instantly fries. (For this reason, my population only has the imported nobles of a population of 49.) I got a siege by the humans recently because so many of their caravans have been destroyed by the lava, but of course, the siege was destroyed by the lava too.
I'm not quite sure whether this is hilarous or horrible.