OS X Native plugin callback functions

I've written a small iOS plugin, and am trying to make an OS X port. In particular, the plugin is for an augmented reality application, so it uses the webcam. Having to build to a device every time I want to test something is slow, so I want to make a version that works in the editor as well (similar to Vuforia).

Since OS X/Desktop doesn't support UnitySendMessage I've been trying to write a simple callback function to replace it.

My native code:

 // .h 
 extern "C" {
     typedef void (*callbackFunc)(const char *);
 }
 static callbackFunc cameraFrameCallback;
     
 // .mm 
 extern "C" {
     
     void _initWithCamera(const char * camera, GLuint texID, GLfloat width, GLfloat height, callbackFunc callback) {
       
         // some other init here
     
         cameraFrameCallback = callback;
         if (cameraFrameCallback != NULL) {
            cameraFrameCallback([@"IS THIS PRINTING?" UTF8String]);
         }
     }
 
 }

C# code:

 public delegate void cameraFrameCallback(string message);
 
 [MonoPInvokeCallback(typeof(cameraFrameCallback))]
 public static void PrintLog(string message) {
     Debug.Log(message);
 }
 
 [DllImport("DesktopWrapper")]
 private static extern void _initWithCamera(string camera, int texID, float width, float height, cameraFrameCallback callback);
 
 public static void InitWithCamera(string camera, int texID, float width, float height) { 
     if (Application.platform == RuntimePlatform.OSXEditor) {
         _initWithCamera(camera, texID, width, height, PrintLog);
     }
 }

When I call the init method, I would expect the callback to get registered then send back the call to PrintLog, which would print my message in the Unity console. However, when I run this, I get nothing back. My plugin is actually running though, because I'm getting camera frames back as I would expect, just the callback function isn't firing.

Am I missing something obvious, or does this just not work in the Editor?