Hi

As far as I know body content rewriting it's not possible with nginx, I may be wrong, i'm a newbie with nginx.
What nginx supports is URL rewriting through HttpRewrite module and this has nothing to do with returned body content rewriting.

URL Rewriting works when nginx receives a requests, it parses the URL requested and rewrite as appropiate according to your rules. At this point the call have not been made to the real request. It doesn't matter where the real call (after rewriting is done) is then sent, be it reverse proxy, static, fcgi daemon, whatever you want.

What do you want is to rewrite the content that the backends returns, that's not something you can do with the HttpRewrite module.

What indeed you can do to solve your problem is to setup nginx to accept ssl connections for that site, let the login form send users to ssl and inside ssl server config in nginx place a 301 redirect to your http server.

Cheers

Guzman