Trying to include a transformed App.config in a XAP file - silverlight-4.0

Question

I have a Silverlight project with multiple configuration files, and am using the transformation approach shown here:
App.Config Transformation for projects which are not Web Projects in Visual Studio 2010?
This approach doesn't work as-is for Silverlight projects though. I've re-written the MSBuild project to look like this:
<ItemGroup>
<None Include="App.config" />
<None Include="App.QABuild.config">
<DependentUpon>App.config</DependentUpon>
</None>
</ItemGroup>
....
<UsingTask TaskName="TransformXml" AssemblyFile="$(MSBuildExtensionsPath)\Microsoft\VisualStudio\v10.0\Web\Microsoft.Web.Publishing.Tasks.dll" />
<Target Name="BeforeCompile" Condition="Exists('App.$(Configuration).config')">
<!-- Generate transformed app config in the output directory -->
<Message Importance="high" Text="Transforming 'App.$(Configuration).config' to output config file..." />
<TransformXml Source="App.config" Destination="$(OutputPath)App.config" Transform="App.$(Configuration).config" />
<ItemGroup>
<Content Include="$(OutputPath)App.config" />
</ItemGroup>
</Target>
<Target Name="BeforeCompile" Condition="!Exists('App.$(Configuration).config')">
<Message Importance="high" Text="Using default 'App.config' as output config file..." />
<Copy SourceFiles="App.config" DestinationFiles="$(OutputPath)App.config" />
<ItemGroup>
<Content Include="$(OutputPath)App.config" />
</ItemGroup>
</Target>
This code generates the correct output file for the correct configuration, however it is never included in the XAP file, even though I am putting the output config into the Content item group. All I need to happen is for the output config to get included in the output XAP but I can't get this to happen.
Can anyone tell me what I'm doing wrong? I'm not an MSBuild expert by any means!

Answer 1

Found the solution by digging into the Silverlight 4 targets. Turns out the XAP packager target actually takes an item called XapFilesInputCollection, which is where the input files come from. The Content item looks likes it is copied to this item before my target runs, so modifying the Content item afterwards is the wrong approach.
All I did was add the transformed files directly to the XapFilesInputCollection item and it worked as I expected.