On the map that was posted a few weeks ago with the undersea volcano, I'm working on a pump stack to make the undersea volcano "erupt" and make my own obsidian island. Almost had the pump stack ready too when a Draltha or Blind Cave Ogre or something got into it and broke a bunch of pumps and nearly started a tantrum spiral by killing the dwarves that were near it. Also, there's the still the teensy problem of how to power 100 pumps on a map where the ocean is frozen... (I have to start the eruption before I can turn temperature back on, otherwise the water falling in the volcano will slow my FPS to nothing and plug up the volcano.)
Anyhow the build-your-own approach still requires a tiny strip of land be present at the corner of the map to begin the embark, so it's not really isolated. I'd love to see a world-gen parameter in future versions like "Generate tiny islands? (Yes/No)". Instead of jiggering around with elevation change rate to try to make tiny islands appear, it would be awesome if a step of world gen actually made these islands explicitly, like through actually making a volcanic island before putting trees and animals on it. Especially once Toady gets around to adding boats and letting you control/watch your wagon during embark. Can you imagine how awesome it'd be if your wagon was really a boat and you could get shipwrecked on one of these islands during an embark?