Perhaps you missed the second point in that post.
It begins with: "For apps that require processing between a hostnode-mapped domain and the main-domain"..
Also, see my previous post to Eduardo regarding building return_url in ad_get_login_url.
You should be able to build a proc that returns the absolute reference you need, without messing with the request processor or redirecting.
Are you trying to hide a form processing url from the web? If so, put the form processing in an index.vuh page. No redirect required.