I understand you scenario. Unfortunately, we don't support it out of the box, but it could be implemented. I would suggest you to handle the INotifyLayoutChanged.LayoutChangeStarted event and to implement your logic in the handler.
Please, refer to the following link - http://www.telerik.com/community/forums/wpf/docking/slidein-slideout-or-hide-show-event.aspx
The forum thread is for WPF, but it's the same idea for Silverlight.
the Telerik team