Funny, I recently tried exactly the same just to get rid of the need to enter a portnumber. Although I only tried as reverse proxy, haven't tried the in-process route using WSGI
Here's what I did
in httpd.conf made sure mod_proxy.so and mod_proxy_http.so are uncommented.
in mine mod_proxy_rewrite.so was already uncommented so do not know if it is required
then in the extra folder I modified http-proxy.conf and added these lines
and that was all, restarted Apache and now if I navigate in Stanza to localhost/opds I get to see Calibre's OPDS output and if I use localhost/browse in a browser I can use the web-interface and if I want to use web-interface on my phone I visit localhost/mobile
In that same file is likely a section <proxy *>
I've changed that from the default restrictive order deny, allow to
Order allow,deny
Allow from all
for testing purposes, so make sure to modify this to your requirements if you restrict access to only selected users.