You can put your $PROJECT_HOME anywhere you wish. According to the documentation, you can :
Create a virtualenv and a project in the same time using
mkproject [-t template] [virtualenv_options] ENVNAME
"Bind" an existing project to an existing virtualenv
setvirtualenvproject [virtualenv_path project_path]
You will find all the details here :
http://virtualenvwrapper.readthedocs.org/en/latest/command_ref.html#project-directory-management
Virtualenvwrapper is so easy to work with !