Seems I've run into a bit of a snag with dwarven spiders - they're being marked as "friendly," which isn't allowing pen/pasture, tame, train, or any other designations.
... Which means I'm going to need to use a pretty elaborate workaround to get them into position, involving a new building and an "intermediary" transformation. In order to make them friendly, I need to transform a dwarf, force it to drop a body part, allow it to revert, animate it, transform it, kill it, resurrect it, and transform it again. This actually works, believe it or not, but the problem is the inability to pasture them.
I tried a few workarounds.
Apparently, animated creatures are ridiculously weak, and collapse under any sort of physical damage... and also seem to inherit the OPPOSE_TO_LIFE tag, even if it isn't explicitly given to them. So for my first test run... I had a dwarf create a a spider which immediately went on a rampage and killed 4 dwarves before being struck down by a sabre-toothed tiger. Revise, restart.
This time, I give it a speed percentage effect so that the tiger can kill it without interference. It dies, and I give the dwarf an ability to revive it, and now it turns friendly. Problem is, it won't leave the general area around the workshop.
Next time, I try caging it, but this doesn't seem to make it register as a captured creature either.
It looks like I am going to have to create a new building called a "dwarven spider hub" and move the assembly reaction there, so if you want to station spiders somewhere, you'll need to build that little building and assemble them there, and they'll use that as their permanent base... I figure it fits into the lore pretty well - seeing as these are the first and most primitive of constructs, their pathfinding and navigation skills would be comparable to a small insect. I can even make it so that when they die, you can attempt to rebuild the little buggers. Perhaps I can make later constructs and advancements utilize a sentient being as their intelligence core, which would be far more effective in that it would allow you to train, assign, and pen/pasture them. I like that idea, so that's what I'm going with.
This is why I insist on testing these things first, though, in case you all were wondering. !!SCIENCE!! is fun, but nobody likes it when I release something and the features don't work right.