I have some news. After digging around, I finally got the RadTabItem for the currently selected item.
RadTabItem tabItem = container.ItemContainerGenerator.ContainerFromIndex(container.SelectedIndex)
But even with this, it's not getting focused "visually". If you check the IsFocused property, it is set as true as it should be, but nothing happens. Before databinding the ItemsSource of the RadTabControl, I was creating new RadTabItems and adding them to the RadTabControl.Items property, and giving them focus worked :(
Any ideas? Thanks again!