How to include an item in BuiltProjectOutputGroup - msbuild

Question

I have a custom project system, that uses the standard net sdk targets.
During the build, I produce an extra zip file. I'd like this extra file to be included in an output group, so that when I query my projects output groups (from vs) it shows up.
My project file looks like this:
<Project Sdk="Microsoft.NET.Sdk">
... stuff
<ItemGroup>
<PackageReference Include="DnnVsProjectSystem.BuildTools" Version="0.0.5">
<PrivateAssets>All</PrivateAssets>
</PackageReference>
</ItemGroup>
<Import Project="$(CustomProjectExtensionsPath)DnnVsProjectSystem.targets"/>
</Project>
Notice, I am using the "sdk" attribute, which is a fairly new feature of msbuild.
The PackageReference that you see, is a nuget package that imports a .props and a .targets which augment the build with some custom build tasks. These are the ones that produce the zip file.
I have drilled into the net sdk targets and found this:
<Target Name="AllProjectOutputGroups" DependsOnTargets="
BuiltProjectOutputGroup;
DebugSymbolsProjectOutputGroup;
DocumentationProjectOutputGroup;
SatelliteDllsProjectOutputGroup;
SourceFilesProjectOutputGroup;
ContentFilesProjectOutputGroup;
SGenFilesOutputGroup" />
<!--
This is the key output for the BuiltProjectOutputGroup and is meant to be read directly from the IDE.
Reading an item is faster than invoking a target.
-->
<ItemGroup Condition=" '$(OutputType)' != 'winmdobj' ">
<BuiltProjectOutputGroupKeyOutput Include="#(IntermediateAssembly->'%(FullPath)')">
<IsKeyOutput>true</IsKeyOutput>
<FinalOutputPath>$(TargetPath)</FinalOutputPath>
<TargetPath>$(TargetFileName)</TargetPath>
<COM2REG Condition="'$(RegisterForComInterop)'=='true' and '$(OutputType)'=='library'">true</COM2REG>
</BuiltProjectOutputGroupKeyOutput>
</ItemGroup>
<ItemGroup Condition=" '$(OutputType)' == 'winmdobj' ">
<WinMDExpOutputWindowsMetadataFileItem Include="$(_IntermediateWindowsMetadataPath)" Condition="'$(_IntermediateWindowsMetadataPath)' != ''" />
<BuiltProjectOutputGroupKeyOutput Include="#(WinMDExpOutputWindowsMetadataFileItem->'%(FullPath)')">
<IsKeyOutput>true</IsKeyOutput>
<FinalOutputPath>$(TargetPath)</FinalOutputPath>
<TargetPath>$(TargetFileName)</TargetPath>
</BuiltProjectOutputGroupKeyOutput>
</ItemGroup>
This appears to be the target that is called by VS, when it wants information about output groups.
The problem is, i am not sure how I can get my item included in one of those output groups, as If i just add the item to the item group, in my own targets - my targets are irrelevent at this point, as they are not included in this dependency chain.
I also can't override any of the targets, because, as i'm using the sdk attribute, it looks like the sdk targets will always be imported last, overwriting anything that I declare.
Any guidance much appreciated.

Answer 1

If your only concern is to hook into the target or its dependency chain, I suggest using msbuild's BeforeTargets functionality:
<Target Name="IncludeMyCustomOutputGroup" BeforeTargets="AllProjectOutputGroups" DependsOnTargets="ResolveMyCustomPropertiesAndItems">
<ItemGroup>
<!-- Assuming #(MyCustomOutput) items are generated by your ResolveMyCustomPropertiesAndItems target, or just add anything else -->
<BuiltProjectOutputGroupKeyOutput Include="#(MyCustomOutput->'%(FullPath)')">
<IsKeyOutput>true</IsKeyOutput>
<FinalOutputPath>$(TargetPath)</FinalOutputPath>
<TargetPath>$(TargetFileName)</TargetPath>
</BuiltProjectOutputGroupKeyOutput>
</ItemGroup>
</Target>