How much, exactly, can one change at the caste level? Though I don't really want to give too much about my plans away, would it be possible to, for example, change things like body structure from one caste to the next? Or, I don't know, replace one (but only one) caste's bones with solid gold, or something?
both are fully possible.
Also, you can add castes pretty much no matter what they change. The reason Genesis crashed is because male and female castes have different bodyparts. Also, messing with already-existing castes is no issue as long as you don't change their bodies, I figure.
When it comes to changing bodyparts at the caste level, both REMOVING and ADDING are equally difficult, since you need to define the entire body at the caste level. Also, if you define the body at the creature level and re-define the body at the caste level, you get 24-armed 4-headed freaks. Just a warning.
(I know a shitload about caste modding at this point because it's pretty much the only creature modding I do
)
For an example of just how much you can change through modding, see my "friendship" (in the DFusion sense) mod:
http://dl.dropbox.com/u/13790475/creature_dwarf.txt