Interestingly enough, I have reverse proxy working, with the following config.
I've obscured the port numbers. outside_port_number is the port number my server shows the outside world; inside_port_number is the one that calibre_server uses.
Code:
Listen outside_port_number
<VirtualHost *:outside_port_number>
<Proxy * >
AuthType Basic
AuthName "Book Repository"
Require valid-user
AuthBasicProvider file
AuthUserFile /path/to/authorized_users
</Proxy>
ProxyPass / http://localhost:inside_port_number/
ProxyPassReverse / http://localhost:inside_port_number/
</VirtualHost>
From the outside I can use basic authentication, which then requires username and password.
Unfortunately, even with basic authentication enabled, neither Aldiko nor WordPlayer will authenticate to Calibre.