E.g. I build a new virtualenv from scratch for each deploy, so that would mean that the postactivate script would need to be accessed from somewhere by the build process.

I was talking about local dev install where you can activate postactivate script is run after you source activate or call workon. In prod, you will not be activating the env but running executables directly from the env.

ie, your process manager will call /path/to/env/bin/gunicorn and in this case, you'll have to configure the env variables here (in the process manager (supervisor)) config.