I want the environment variable DJANGO_SETTINGS_MODULE to change depending on what I say for workon. It seemed to me that I want to set it in .virtualenvs/postmkvirtualen
You were on the right track, but you want to use the postactivate hook instead of postmkvirtualenv.
It won't restore automatically on deactivate. Thankfully there is postdeactivate hook that you can use to manually restore any environmental variables that you changed on activate.