I seemed to have found an evolution of the answer from @John Hunter that is much simpler, add this to the csproj.
<ItemGroup>
<Content Include="Bin\**\*.rpt" />
</ItemGroup>
This will then make visual studio automatically view all *.rpt files in that folder as part of the solution. You could go with *.*
to accumulate everything. This makes more sense if you have a container folder like bin\MyDeployables\**\*.*
We followed a similar usage for using Cassette MSBuild to combine and minifiy our JS at publish time, and be able to publish the created files through the built in VS publish tooling.