Google 2 step authentication has a pam module:

http://code.google.com/p/google-authenticator/wiki/PamModuleInstructions

So, theoretically, it could be implemented into nginx. What do you think about it?

My goal is to have 2 step auth, somehow, on some dir of my web server (some directory also has static content, so it isn't a php script i can edit)

I think that it would be interesting. Many thanks!