To answer my own question, in case someone who reads this is also considering scripting possibilities.
So it seems (most?) Archicad data (from favourite components or others) can be read/changed after all.
With a C# script, if you make sure you avoid an (implicit) conversion (from the original Archicad class to the generic Grasshopper class) while getting the data, then you GetType(), and then with intellisense/autocomplete you can see what's inside the Archicad class, including parameters you can change by script.
I have ended up writing a small number of helper functions (creating/deleting a component on the grasshopper canvas, connecting/disconnecting objects, getting/setting data, etc). It's basic functionality, but it takes quite a bit of time if one is not familiar with the Rhino/Grasshopper API, or also C#. I could attach them here at some point, if there is an interest.
I'm not sure if Python is a good option for the above, I didn't give it a proper chance, but the dynamic typing and also the inferior intellisense created problems for me.