Where is the scoped Prism RegionManager for my tab control?

Question

Using Prism 5.0. My shell is instantiated from a MefBootstrapper derived class. The shell has a MainRegion where views are added using the AutoPopulateExportedViewsBehavior (found in the StockTraderRI sample). This works perfectly.

However, in one of my views that is loaded into the MainRegion, there is a TabControl where I want to add active-aware viewmodels in runtime. The IActiveAware interface only works if the control is inside a region, and a region does not allow me to bind to ItemsSource. Hence this construct:

<TabControl prism:RegionManager.RegionName="TabRegion">
    <TabControl.Resources>
        <DataTemplate DataType="{x:Type local:MyTabViewModel1}">
        </DataTemplate>
        <DataTemplate DataType="{x:Type local:MyTabViewModel2}">
        </DataTemplate>
    </TabControl.Resources>
</TabControl>

So I thought that I could add the viewmodels in runtime, when my main viewmodel is activated (via IActiveAware):

protected override void OnActiveChanged(bool active)
{
    if (active)
    {
        // This code crashes.
        // The region does not exist in the root region manager...
        var region = regionManager.Regions["TabRegion"];
    }
}

The problem is that this code crashes. I guess this means that I have a reference to the root region manager. How do I get hold of the scoped region manager?