Let me start with my apologies for the delayed reply.
In fact this can not be achieved with using the TabStripPlacement property of RadRibbonTab as it is read-only. However you might tweak the RibbonView style and rearrange its elements so that the RibbonScrollViewer (theone that wraps the Tab Items) is outside TopGrid element and below the ContentPresenter that holds the SelectedTabContent. I'm attaching a project where this is demonstrated made with our VisualStudio2013 theme and Implicist Styles
theming approach. Please have in mind that it requires you to reference NoXAML binaries as described in the same help topic mentioned above. Feel free to modify the solution per your own scenario needs.
P.S. The project is made under WPF but the approach is practically the same for Silverlight. Just make sure that you use the correct implicit style (the one made under Silverlight) as both platforms differ in XAML.
Check out the Telerik Platform - the only platform that combines a rich set of UI tools with powerful cloud services to develop web, hybrid and native mobile apps.