Hm... it shouldn't, no. Only global_env is copied, so Arc types wouldn't need to worry about copying. In the hypothetical case that I would need to copy them, I could coerce them into a table and then do a copy on that. This could all be done transparently, so Arc code wouldn't need to do anything.
global_env is treated specially because it's the base for all other modules: it contains the built-in functions that modules cannot create (at least not in Arc), and also the core functions in arc.arc for convenience.
There are other strategies I could use as well, like special-casing (assign) or similar, but I figured a shallow copy would be easiest.