Hi Scott, now that mod_plsql is about to be deprecated, I am again testing ORDS v3 / Tomcat without Apex. So far it's working. I am not quite ready for deployment as there are a few details to iron out. Over all ORDS is pretty impressive.
As for putting Apache in front of ORDS as shown in your image, I am pretty sure all that's needed is a ProxyPass in Apache. Add this at the end of the httpd.config file:
ProxyPass /ords http://localhost:8080/ords
ProxyPassReverse /ords http://localhost:8080/ords
Similarly, the same principle holds true with Ngnix. This would be a good place to load balance between a few ORDS instances and SSL terminate.
According to some of the Tomcat bloggers putting Apache in front of Tomcat may not be too necessary. In Tomcat 5.5++ the same native/APR connector exists as Apache's Portable Runtime APR and the performance is similar. http://www.tomcatexpert.com/blog/2010/03/24/myth-or-truth-one-should-always-use-apache-httpd-front-apache-tomcat-improve-perform