Here's an interesting bit: There is actually a way to replace a creature's tissue material with another material directly. For instance, you can create a tool called a '(material) nutrient wad' or something, that can be made out of any metal. Then run a reaction that will turn the worker's exoskeleton tissue into the material the wad is made of. This is how the construct-creature script works (spawning a creature and then changing its material), but it can be just as easily applied to an already existing creature.
Unfortunately there is no simple way to display the changed material in-game (construct-creature uses a rather extensive workaround just to display the proper color), although it is possible to change the creature's first name to something that includes it.
It's really kind of strange that this is possible, it stands out as a glaring flexibility in the normally rather inflexible body structure code. Perhaps Toady was planning on adding in a curse that turns a creature into living metal or something but scrapped it halfway through.
