How can I set HTTP headers to a Portal response to avoid caching at proxy level?

Issue

• The proxy server fronting JBoss Portal caches content. We would like to avoid caching, and have tried to set the response headers by adding the following to $EPP_HOME/server/$SERVER_CONFIG/deploy/jboss-portal.sar/portal-core.war/WEB-INF/jsp/header/header.jsp, however this does not have an effect, the headers are not added as expected:

   response.setHeader("Cache-Control","no-store");
   response.setHeader("Pragma","no-cache"); 
   response.setDateHeader ("Expires", -1);