I noticed you can pass haproxy.cfg into /tmp/haproxy.cfg and it's used as the input template for generating the final haproxy.cfg. This is nice, but unfortunately you're limited in what you can do since the frontend and backend sections are generated based on templates stored directly in the python script.
It would be nice if these templates were moved somewhere else, e.g /tmp/haproxy.d/frontend.cfg, so that I could pass in my own template with customizations.