Conditional default item metadata - msbuild

Question

I want all project files under Content\** to have CopyToOutputDirectory set to PreserveNewest by default while still having to add each item (therefore no wildcard include). Something like:
<ItemDefinitionGroup>
<Content Include="Content\**\*">
<CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
</Content>
</ItemDefinitionGroup>
Unfortunately ItemDefinition does not support Include attribute.
I have also tried:
<ItemDefinitionGroup>
<Content>
<CopyToOutputDirectory Condition="$([System.String]::new('%(Identity)').StartsWith('Content\'))">PreserveNewest</CopyToOutputDirectory>
</Content>
</ItemDefinitionGroup>
as suggested here but it seems like it doesn't work in ItemDefinition.
In fact when I've attempted this:
<ItemDefinitionGroup>
<Content>
<CustomToolNamespace>Foo = $([System.String]::new(%(Identity)))</CustomToolNamespace>
</Content>
</ItemDefinitionGroup>
The value of CustomToolNamespace reported by Properties pane was Foo = %(Identity) .

Answer 1

If you want to create new metadata for your Itemgroup:
<Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003" >
<ItemDefinitionGroup>
<Content>
<CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
</Content>
</ItemDefinitionGroup>
<Target Name="Definition">
<ItemGroup>
<Content Include="Content\**\*"/>
</ItemGroup>
<Message Text="%(Content.CopyToOutputDirectory)"/>
</Target>
</Project>
This adds the new metatdata to the itemgroup.
You could also combine two itemgroups based on different Includes and metadata like:
<Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003" >
<ItemDefinitionGroup>
<WithContent>
<CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
</WithContent>
</ItemDefinitionGroup>
<Target Name="Definition">
<ItemGroup>
<NotContent Include="Content\**\not*"/>
</ItemGroup>
<ItemGroup>
<WithContent Include="Content\**\with*"/>
</ItemGroup>
<ItemGroup>
<Content Include="#(NotContent)"/>
<Content Include="#(WithContent)"/>
</ItemGroup>
<Message Text="Added together: %(Content.CopyToOutputDirectory)"/>
<Message Text="Added together: %(Content.Identity)"/>
</Target>
</Project>