It is not possible as of PTVS 2.0. It simply doesn't run any target on "build" (the fact that it shows "Building" in the Output window is really more of an artifact of VS, which has a mandatory build step for projects). That said, we have an outstanding feature request for this, and recognize the value of scripting various packaging and similar task through MSBuild.
We are not doing this exact thing for the upcoming 2.1 release, but there is something similar that might help you - since "building" does not actually make sense by itself for Python, we are instead allowing you to define commands in MSBuild that are exposed in Visual Studio UI on the project node in Solution Explorer; those commands can be backed by an arbitrary Python script. Have a look at these to see what this looks like (bearing in mind, of course, that this is an actively developed feature, and the code will probably change as we are working on it):
https://pytools.codeplex.com/SourceControl/latest#Python/Product/BuildTasks/Microsoft.PythonTools.targets https://pytools.codeplex.com/SourceControl/latest#Python/Product/Django/Microsoft.PythonTools.Django.targets
(search for <PythonCommand>
)
We don't have a dev build that includes those changes yet to easily play with it, but we should get one out soon. You can, of course, just take the most recent sources and build them.