One way to do this is to use the AfterEndToEndIteration object for your TFSBuild.proj file, which will run the TfsBuild.exe command line to start other assemblies. I think something like this (although I have not tested it)
<Target Name="AfterEndToEndIteration">
<GetBuildProperties TeamFoundationServerUrl="$(TeamFoundationServerUrl)"
BuildUri="$(BuildUri)"
Condition=" '$(IsDesktopBuild)' != 'true' ">
<Output TaskParameter="Status" PropertyName="Status" />
</GetBuildProperties>
<Exec Condition=" '$(Status)'=='Succeeded' "
Command="TfsBuild.exe start /server:$(TeamFoundationServerUrl) /buildDefinition:"Your Build Definition To Run"" />
</Target>