However, he wants to try and cast a layer of farm obsidian befor digging out an area under it for additional casting, with the aim of preventing it from ever losing the Subterranean/Dark tag, which is interesting and, AFAIK, an unknown.
That's it, exactly. However, the wiki says it'll work, so either someone has done it before, or someone was talking out of their ass
I'm hoping it's the first one.
Anyway, my first attempt went not with a bang, but with a whimper. I forgot that magma evaporates, and the whole expanse of the embark area (which is, IIRC, 5x5) is a tad too much to fill with three pumps.
Instead, I'm now moving towards a slightly more sophisticated design, with a bottom chamber all across the world spanning two z-levels, and then two more levels divided in 5x5 squares connected by throwaway doors (meaning I know they'll melt once I open them. No biggie, I've got economic stone up the wazoo, anyway, with the strip-mining). Each of these rooms will have a 3x3 retractable bridge built in order to release their contents (magma for the first level, water for the second).
If I open all the bridges on the first level at the same time, and once that has settled the bridges on the second, I should have a complete obsidian covering of the sky (except for those annoying five tiles near the borders).
It's going to take a frightening amount of bauxite, between the bridges and the mechanisms (only for the lower level, which will contain magma), but there's plenty in the map, which I'll mine once my miners have reached legendary.
I'll keep you guys posted on how well this goes.