Sure; reflection can do what you want.  The naive way would look like
this (omitting exception handling).

   if (System.getProperty("os.name").toLowerCase()
       .indexOf("mac") != -1)
   {
       Class appc = Class.forName("com.apple.eawt.Application");
       Object app = appc.newInstance();

       Class lc = Class.forName(
           "com.apple.eawt.ApplicationListener");
       Object listener = Proxy.newProxyInstance(
           getClassLoader(), new Class[] { lc },
           new InvocationHandler() {
               public Object invoke(
                   Object proxy,
                   Method method,
                   Object[] args)
               {
                   if (method.getName().equals("handleQuit")
                   {
                       attemptToQuit();
                   }
               }
           });

       Method m = appc.getMethod("addApplicationListener", lc);
       m.invoke(app, listener);
   }

Of course, that's the naive way.  A better way would be to put all that
code into a class that implements some interface, and then do something
like this:

   if (System.getProperty("os.name").toLowerCase()
       .indexOf("mac") != -1)
   {
       AppCloseHandler h = Class.forName(
           "mypackage.MacintoshCloseHandler").newInstance();
       h.setupClose();
   }

AppCloseHandler is an interface that you write.  Inside of the class
mypackage.MacintoshCloseHandler, you can make static use of the Mac-
specific classes, since this code will only get loaded when those
classes are available.

I still find it hard to believe that WindowListener.windowClosing
doesn't work properly on the Mac... but if you've tried it, I guess I
can't argue since I don't have a Mac to try it with.

I can't imagine why you're using toLowerCase, since the value of that
property when running on Mac OS X will be *exactly* "Mac OS X" on every
system.

In what way is it failing on Windows?  Do you have code references
elsewhere (not shown here) to those Apple classes that are evaluated at
runtime on a Windows machine?

You might find more help from Apple via some useful web pages at  
<http://developer.apple.com/referencelibrary/Java/idxPorting-date.html>.  
On that page is a link entitled OSXAdapter that shows (with source code)
how to use reflection to hook these Apple classes into an application.  
Just below it is one entitled Unsolicited About Boxes which shows more
on how to properly override the About and Quit behaviors.  And just
below that is a link entitled AppleJavaExtensions where you can get stub
classes for compiling on other platforms.  I would think that one or
more of those would provide useful answers.

= Steve =