I am not quite sure I understand the exact scenario. By design, the content of RadSlidingPane is rendered on the page when the pane loads and can be accessed using the standard ASP.NET approaches - using $get() / $find() methods client-side, or directly accessing the server controls server-side.
If the RadSplitter is placed inside asp:FormView control you need to use FindControl method to access the controls inside it since the FormView control is a naming container, e.g.:
radioButton1 = $get(
'<%= FormView1.FindControl("RadioButtonList1").Items.ClientID %>'
If this is not the case, could you please explain in more details what the specific scenario and what exactly you are trying to achieve?
the Telerik team