IMHO you're better off editing the project file directly and add it to the AfterBuild target. Here's an example where we copy assembly DLL, PDB and XML files on a release build:
<Target Name="AfterBuild">
<CallTarget Targets="CopyFrameworkToTools"
Condition="'$(Configuration)' == 'Release'"/>
</Target>
<Target Name="CopyFrameworkToTools">
<CreateItem Include="$(OutputPath)$(AssemblyName).*">
<Output ItemName="ReleaseFiles" TaskParameter="Include" />
</CreateItem>
<Copy SourceFiles="@(ReleaseFiles)" DestinationFolder="..\Tools\" />
</Target>
You could strip that down to one line using Copy task, but I want to show that integrating into MSBuild gives you more flexibility and control.