According to http://tomcat.apache.org/tomcat-5.0-doc/config/valve.html the access log format supports "%{xxx}i for incoming headers", but there is no corresponding "%{xxx}o for outgoing headers". This is however supported by Apache HTTPD, see http://httpd.apache.org/docs/1.3/mod/mod_log_config.html, so this complicates porting configurations a bit.
Look at Revision: 566319 at tomcat 5.5.x trunk and give my fix a try :-)