If I recall correctly, it's impossible to issue a redirect with POST (see section 10.3 of RFC 2616); in other words, you can't use a Location header to issue a POST redirect. However you can ask the browser to redirect a POST request.

A possible solution is the use of tickets: redirect users to their destination with a "one time code", that the final server can use to verify identity (contacting the login server behind the scenes).

Ciao, Valerio