Is there any programmable/customizable Open Source Proxy server which allows me to do this:
For certain pages which get an authentication page which always looks the same (it's custom, not the standard proxy authentication stuff) I can hook up a script which sends the login credentials (via POST).
The reason is to work around a nasty IT policy which prevents me to use Emacs ELPA packages. Emacs package manager tries to connect to an URL which returns this login page as HTML, and naturally Emacs doesn't know what to do afterwards.