Ah, now I recheck, it seems four of them wore helms, two of them made of paktong, one of ocean steel, and one of bronze, but no metal top hats. I had noticed that the lagomer and formician bards/mercs who visited brought various clothing items made of metal, though. The three hammermen in the siege force had brought cone hammers.
I'm actually using a slightly modified version of dwarves that I usually test the Fortress Defense Mod with, with their personalities reshaped to love fighting, and access to the standard materials and tech of vanilla dwarf civilization.
I think I had an easy time of it because I had been prepping for 3 years, and the siege was only ten individuals, who were heavily outnumbered by my own soldiers. So far I've had only one other siege in the following year, by goblins, but I will continue running the fort and waiting for new invaders.
Edit: Ran into a Formician siege. They were all small, and using metal clothing including novachalcum, aerochalcum, and cloud steel, helmets made of various metals including blaze and ocean steel, and tower shields. Their ranged units were three bowmen, using pyrochalcum and other modded metal bows, and lead arrows. Overall the force was poorly armored.
In the process my own Formician Master Lancer died. While the rest of the army fought off the rest of the formician invaders, she managed to find a coati, who ate her head and ran off with her masterwork spear
It looks like the small size and poor armor of the formicians make them less of a threat than the large races in this mod.