While searching for solutions I learned that mod_proxy has been passing X-Forwarded-For, X-Forwarded-Host and X-Forwarded-Server since version 1.3.25 from the following messages dated 2002 and 2003. So I tried and it worked. The relevant code in mod_proxy begins on line 315 of my copy of apache_1.3.36/src/modules/proxy/proxy_http.c.