I'm impressed by the completeness of this library, I stumbled across it while I thinking about building a CG library built on SDL2.0.
It has a pretty complete UIKit framework, so It should be quite simple to adapt it to include the APPKit framework.
I need to download the current Darling codebase to see where things stand that the moment and see if I have time to contribute.
I am a fairly experienced C and Obj-C coder, but time is a precious resource for me :-)
This is a very worthwhile project.
Matt P.