In an ill-advised bid to combat daylight adaptation, I exposed my mist generator staircase to light. Now the top tile freezes with the onset of winter. I don't know if any poor Urists are trapped in there right now, but I'd like to avoid it happening next year if not.
So how do I deal with this? My first thought is magma, obviously, but the placement doesn't really work.
That aside, I think there are two strategies that could work
1) Temperature control to prevent freezing.
2) Dorf exclusion with tile drainage.
I can't place a fortification on the death tiles to exclude passing dwarves, because that will create a floor and water won't flow through right. Waling them out of the space would be insanely disruptive to fortress design and create a bunch of extra steps.
I can't use vertical bars or a wall grate, because those won't build on empty space.
If blocking the sun were going to work, I don't think it would have frozen in the first place because the whole thing was always under a bridge. But will multiple layers of insulation help? I've considered building a couple Z-levels of insulating walls over where the bridge used to be --sort of like the Chernobyl sarcophagus except backwards.
Maybe nether cap floor grates above and below could help stabilize the temperature?
I guess I could start leaving the space open, but then I'm worried about Dorfs sidestepping into the void and falling to their deaths. Or does that not actually happen?