An OverviewMap shows one layer which is given by the layer property of the OverviewMap. This layer can be set by code or by XAML and doesn't need to be the same than the layers in the main map.<esriToolkit:OverviewMap x:Name="OVMap" Margin="0"
MaximumExtent="-180,-90,180,90" Map="{Binding ElementName=Map}">
<esriToolkit:OverviewMap.Layer>
<esri:ArcGISTiledMapServiceLayer Url="http://server.arcgisonline.com/ArcGIS/rest/service/ESRI_StreetMap_World_2D/MapServer" />
</esriToolkit:OverviewMap.Layer>
</esriToolkit:OverviewMap>
So I guess you could set the overviewmap layer to your graphic layer./Dominique