In order to get the RadPane in which your custom UserControl is placed all you need to do is in its Loaded event cast the sender as your custom UserControl and then access the RadPane from the Parent property of that UserControl.
The next code snippet shows how the Loaded event of your custom UserControl should look like:
sender, RoutedEventArgs e)
var myUserControl = sender
var pane = myUserControl.Parent
Hope this is helpful.
the Telerik team