What living creature would the tarantulas be needing? Also, could I not pasture 50 creatures in the workshop, start and finish one reaction so they all get caught in the vapor and get transformed through one reaction?
How I was planning to do it was to grant an interaction to the dwarf performing the reaction (via syndrome) to transform a single creature. So, basically, it gives him an interaction to transform one nearby creature which has a cooldown sufficient enough that it can only be used once before the ability to do the interaction expires. As for which creature this will require... I'll get back to you on that one.
One could attempt to put multiple dwarves on top of the workshop to get more than one to receive the benefit, but there really isn't much I can do about that. Conceivably you could abuse burrows, but there isn't much I can do to stop that. I considered giving the interaction via a special alcohol, but then you'd have very little control over.... wait. Heh, I just got an idea.
Disregard the previous paragraph, I'll just grant the syndrome via a single unit of alcohol. The interaction will not require LOS or have a max range, but can only be used once per dose. I'll probably be introducing a new tame creature to serve as a platform for this transformation, which you'll need to either breed or import more of. Yeah, I'm sticking with that story for now.
... And I just got an idea for a diabolical new creature. Hehehehehehehe... More on that tomorrow, if I get time between class, a doctors appointment, and playoff hockey tickets.