Some ideas,

1. We add any possible UI apps to the main requirements file.  I.e. we put KoBo's UI in a separate repo and pip library (or git repo), then add it to requirements, it gets installed always but changes can be made independently and is only active in specific sites

2. In the fab deploy script we read a local setting for `TEMPLATE_REPO` or something analogous, and pip install it.  We assume some convention, like there must a pip installable library with the same name, or there must be a github repo with the same name, or maybe there's no additional constant and we assume there's a pip/repo that's` '%s-templates' % SITE_NAME`.