// Try some crazy reflection bullshit
try
{
    // Grab the class
    Class applicationClass = Class.forName("com.apple.eawt.Application");

    // Grab the methods
    @SuppressWarnings("unchecked")
    Method getApplicationMethod = applicationClass.getMethod("getApplication");
    @SuppressWarnings("unchecked")
    Method setDockIconMethod = applicationClass.getMethod("setDockIconImage", java.awt.Image.class);

    // Call getApplication()
    Object macOSXApplication = getApplicationMethod.invoke(null); // Static
    // Call setDockIconImage()
    setDockIconMethod.invoke(macOSXApplication, iconImage);
}
catch(Exception e)
{
    // If it doesn't work, don't worry about it
}