Thank you for contacting Telerik Support.
Please find attached a sample project that implements very similar scenario - when user clicks on "Hide on server side" button will hide the middle RadPanelBar item. Then if user clicks the button with text "show on client side" will show that item again on the client-side without any post-back to the server.
Hope that this will be helpful.
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to the blog feed