1. If you "just" want to map a hostname, to a private IP, you can assign the hostname to a variable, and use the variable in your backend config. This works because Nginx resolves static addresses at start, but resolves names pulled from variables at runtime.

2. If you need to map ports as well, you can use a Lua or Mruby script. E.g. I have a blog post on doing it with mruby here [1], and it's run production sites for a couple of years. This options lets you integrate against pretty much whatever you want.

It's extra hassle, though, of course.

[1] http://hokstadconsulting.com/nginx/mruby-virtualhosts