... What's the appropriate place to ask odd questions about icecasting? (It'd be like obsidian casting, but to get natural ice.)
Hm, I've never tried it, but this might work:
Generate your world with at least 20 "Z Levels Above Ground" in advanced worldgen.
Use only one cavern, with at least 10/15 for "Cavern Layer Water Min/Max"
Dig down to the first/only cavern where there is water.
Make some buckets.
Designate the cavern water as a water source.
Designate an area on the surface to be a pond.
Theoretically, the dwarves should haul the liquid water up to the surface bucket-brigade-style, where it should more or less freeze instantly into ice? Then all you need to do is dig up from underneath some stairs, and continue building upwards to get your enormous ice cube.
It might not work if the water freezes in the bucket on the way up. Dunno!
I tested this with dfhack liquids and it works if you place water 2/7 on the surface you instantly get an ice wall with a -36 worldgen (9937 U probed) temp embark. It takes time, though, when placing water on a large surface like this. It takes many seconds for the water to freeze.
The dwarves are fine at this temperature, never fear! In a vanilla DF 34.11, where I did this testing, the dwarves (and their anmials) were on the surface for at least a month in 9937 U with no ill effects.
After more testing, using 1/7 works, it just takes much longer, and the ice doesn't freeze in big blocks (16x16) but instead in small blocks (single tiles) and multiple layers of water are required. 2/7 works much more predictably. You still have to build in layers, though, or the water has a chance to run a bit and ruins the nice sheer ice cliff face.
Here's what I was able to accomplish in a few minutes:
I had dug a tunnel into the ice wall, then designated some stairs upwards to see if that would work, and it works fine. Let me know if you want more detailed instructions, but it basically boils down to dfhack liquids 2/7 , build one layer at a time, and you can have your ice cliff !