PL/SQL web app: set http response header

How can I set an http response header within pl/sql using mod-plsql? There are certian, very specific, response headers that owa packages support (setting cookies, redirect, mime header/content type). I need to set a Surrogate-Control header to trigger an ESI surrogate. I cannot find either a specific prodecure to set this header, nor a generic set-response header procedure. Any ideas would be useful. Without this ability, it seems very difficult to effectively use ESI in PL/SQL web apps.

--Tim