Ah, okay. Good to know - I gave up on that fort in favour of one that was a bit more forgiving(a 30-tall magma pump stack instead of 130, among other things), but definitely good to know about that little quirk.
Speaking of magma - I searched and found mention that it was unclear which boundary tiles would permit a blast furnace to be magma-fired. Has there been any new development on that front? I can try to slice across it vertically, one tile away from center, and hope that I can get the necessary tile while staying under the impassable tiles(trying to avoid accidental tumbles into magma), but if I can work with a little more precision, it'd certainly be welcome.
ADDENDUM: Seems I've answered that for myself, at least partially. When making a magma blast furnace, the second tile from the left and second from the top(that is, if the top left corner is (0,0), tile (1,1)) is the "anchor" tile, the tile that must be built on solid ground rather than over open space. I would gather that it's the same for other larger-than-normal workshops. At any rate, I have the top middle(2,0) and bottom middle(2,4) tiles dug out, and the blast furnace works; my guess is that the top one is responsible - that any of the eight tiles surrounding (1,1) would work. (2,0) happens to be one of those which is impassable on the finished workshop, which is a handy place to put the channel.