It doesn't seem that you've grasped the full scope of the issue.  The button collection seems to be unmodifiable from any event or code after it has been loaded.  NOT JUST A CLICK EVENT.  For example, the following code demonstrates adding a StackButton on a 5 second delay.  This will also crash the FishEye when the button is clicked.

        public MainPage()
        {
            InitializeComponent();

            AddStackButton();
            AddStackButtonWithDelay(5);
        }


        private void AddStackButtonWithDelay(double delayDuration)
        {
            Storyboard toolbarDelay = new Storyboard() { BeginTime = TimeSpan.FromSeconds(delayDuration) };
            toolbarDelay.Completed += (s, e) => AddStackButton();
            toolbarDelay.Begin();
        }


        
        private void AddStackButton()
        {
            WebFishEyeButton button = _MainToolbar.FindButton("cmdTools");

            if (button.StackButtons == null)
            {
                button.StackButtons = new ObservableCollection<WebFishEyeStackButton>();
            }

            string stackText = string.Format("StackButton{0}", _StackCounter++);

            WebFishEyeStackButton stackButton = new WebFishEyeStackButton();
            stackButton.ItemName = stackText;
            stackButton.Text = stackText;
            stackButton.ImageSource = new BitmapImage(new Uri("Resources/toolbox.128.png", UriKind.Relative));

            button.StackButtons.Add(stackButton);
        }