Yeh you could say that. A basic window with keyboard, mouse and an OpenGL context needs to be written in Cocoa. This should have all the features of the Display, Mouse and Keyboard classes which isn't much and mostly about matching the Cocoa API's to the Display ones. The features of the above should be connected to Java via JNI, there might be a little fiddling needed here with the Java thread and Cocoa thread to get it to work but should be easy if you're familiar with both.
There are a tiny amount of API's not available in Cocoa (e.g. grabbing the mouse) and are currently done in the LWJGL codebase using a bit of Carbon, this code can pretty much be copied over to any new implementation to preserve those features.
Lastly we'll probably need to add/keep support for the JAWT API that uses a CoreAnimation Layer as detailed in this thread
. This won't be used at all for standalone LWJGL application but needed for allowing LWJGL to be embedded in AWT (Display.setParent() and AWTGLCanvas, e.g. for allowing LWJGL use in Applets and other AWT/Swing Applications). This is already implemented in the current codebase (was added to support plugin2 applets) but could use a little further looking at and polishing up.
So not too difficult a task if you are familiar with Cocoa, Java JNI and some C but will require some time and effort to implement.