Yup, darwin exists and is used in shared x11-based environments. Yup, third-party products (gnome enviroment, for example) make calls to ConsoleKit and are ported to darwin. Nope, ConsoleKit doesn't support it:( So this is a feature-request to implement a ck-sysdeps-darwin.c for it.
At first blush, ck-sysdeps-freebsd.c might be a good starting point, since darwin does have libkvm support.
Sure. I'd be happy to accept a patch for that. I can't really test it myself though.