I am not sure about the intended maximum height of the Iron half arch, but I am quite sure it should be possible to build it on top of half pillars.
The intended height might be 9 tiles just like the brick arch.
The maximum height for the iron arch piers was set rather low at 1 tile above grade so you should be able to put it on one layer of a thin half pillar. I remember this being deliberate, though I can not remember exactly why. It may have simply looked disproportional any higher, or may have been for balance.
You can put it on a "thin half pillar" (both stone and brick) however not being able to put it on a "half pillar" was an oversight. I will PR a fix once I have a chance.
Quote from: PJMack on April 05, 2022, 02:45:22 AMYou can put it on a "thin half pillar" (both stone and brick) however not being able to put it on a "half pillar" was an oversight. I will PR a fix once I have a chance.
Fixed.