How to know the active control within XAML designer?

Hello Mike,

The entire design time logic of all controls is separated in a different assembly (a design time assembly). These assemblies are generated via a DesignTime project which is shipped with our SourceCode and you could take a loot at the DesignTime implementation of the RadTabControl (Telerik_UI_for_Silverlight_Source\Controls\Navigation\Design). Making design time project for your control will allow you to plug in such custom logic and controls visible only in the designer for your custom control. You could take a look at this article describing how you could debug/create design time project.
Kind regards. Another useful article is the one for the feature providers and feature connectors used to link the design time logic and the control.

The RadTabControl knows its selection as it its DesignTime project has a feature which derives from "SelectedDesignModeValueProvider" which controls such selection logic.
As for your other question "Or, is there a styling I could apply to the TabControl that would change tabs to breadcrumbs and allow a transition between tab contents?" I am not sure that I understand you correctly as I do not have enough information about the control and the desired design-time behavior.

I hope this information helps

Kiril Vandov
Telerik