ItemsControl DataTemplate when using an arbitrary UserControl collection - wpf

Question

Context
I am working on a all-in-one virtual desktop for my users where most tasks require input through a modal dialog. Some super users can navigate through many kinds of configuration dialogs, which leads to serveral occurences of 2nd level dialogs (the first modal dialog invokes another one). In compliance with internal design orientations, each of those dialogs are placed on top of a semi-transparent grayed out background.
Trying to avoid writing the gray-background-panel over and over, I figured I could use an ItemsControl in order to stack my dialogs. My proof of concept worked wonders using a collection of String. So far so good.
The problem
Things get odd when using a collection of UserControl as the ItemsSource. WPF displays the actual UserControl instead of the ItemTemplate. It feels like the template is not even used when the collection's items are UIElements.
Xaml
<ItemsControl ItemsSource="{Binding SomeList}">
<ItemsControl.ItemsPanel>
<ItemsPanelTemplate>
<Grid />
</ItemsPanelTemplate>
</ItemsControl.ItemsPanel>
<ItemsControl.ItemTemplate>
<DataTemplate>
<Grid Background="#AA000000">
<Label Content="Does it work" />
</Grid>
</DataTemplate>
</ItemsControl.ItemTemplate>
</ItemsControl>
Code Behind
Public ReadOnly Property SomeOtherList As UserControl()
Get
Return New UserControl() {New MyControl}
End Get
End Property
Public ReadOnly Property SomeList As String()
Get
Return New String() {"One item"}
End Get
End Property
The actual question
Is there any way to specify a template when the ItemsSource items already possess one? Going further, can we even template a wrapper arround UserControl while leaving the actual UserControl untouched?
I know the whole thing could be cheated using code behind, but relying on VB or C# would not go through code review unnoticed. We're looking for an XAML solution here.
P.S. I am open to new solutions as long as there is a single, unified way to invoke an arbitrary number of dialogs.

Answer 1

After two days of tests I admited the initial approach was not possible, or at least to the extend of my knowledge. I came up with a decent wrapping solution.
Wrapper
<UserControl x:Class="HolderPopup"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
<Grid Background="#AA000000" HorizontalAlignment="Stretch" VerticalAlignment="Stretch">
<Border Style="{StaticResource ResourceKey=borderBase}" />
<Grid Name="placeHolder" Margin="10" />
</Grid>
</UserControl>
The wrapper's code behind has a constructor to which you pass a UserControl to be placed in placeHolder's children, which lets you use it as shown below:
Usage
Private Shared _popup As ObservableCollection(Of UserControl)
Private Shared ReadOnly Property Popup As ObservableCollection(Of UserControl)
Get
If _popup Is Nothing Then _popup = New ObservableCollection(Of UserControl)
Return _popup
End Get
End Property
Public Shared Sub ModalPush(item As UserControl)
Popup.Add(New HolderPopup(item))
End Sub
Public Shared Sub ModalPop()
If Popup.Count > 0 Then Popup.RemoveAt(Popup.Count - 1)
End Sub
'For WPF binding
Public ReadOnly Property PopupRef As ObservableCollection(Of UserControl)
Get
Return Main.Popup()
End Get
End Property
Any event handler, anywhere in the application can call Main.ModalPush in order to stack a modal window on top of whathever was already there.
While this solution respects the constraints (unified popup handling without forcing some hackish dependency in my popups) I am not entirely satisfied. I feel it should have been possible through templating, which would have the benefit of removing this new wrapper class. All in all, that's an alternative, but not exactly what I was looking for.