As far as I understand you need to display some additional information inside the TabStrip area of the RadTabControl. This could be achieved by customizing the corresponding Template used by the control. Additional information about Editing Control Templates you can find in this article.
I created and attached in the sample project a file called "TabControlStyleResources.xaml" which describes all styles and templates used by the RadTabControl. In order to use these styles you have to explicitly define the file as a MergedDictionary Resource.This could be done this way: