This can be done with lua-nginx-module [1] and its 'balancer_by_lua' directive [2].

There are projects like the one I'm working on [3] that implement dynamic proxies using OpenResty.

[1]: https://github.com/openresty/lua-nginx-module [2]: https://github.com/openresty/lua-nginx-module#balancer_by_lu... [3]: https://github.com/3scale/apicast/