Nginx reverse proxy with custom routing logic

Hi,

In my case the logic about what server should serve the request is not based upon the URL. Instead it based upon a id which may be passed as URL parameter or as a header (encrypted form). If it possible to achieve this kind of routing using Nginx. If yes, how?

P.S: I think what I need is the ability to overwrite the routing code so as to make use of the id passed in URL param or header.

Thanks in advance for helping out,
Ankit