The trick is to use a RewriteRule and mod_rewrite to basically stop Auth_memCookie from being needed for /auth/ logins, which is where the PHP code will be executed to perform the authentication. The 'L' flag in the RewriteRule will stop the rest of the rulesets from executing.
RewriteRule ^/auth/(.*)$ /auth/$1 [L]
The ProxyPass is used to proxy traffic to an internal webserver host, and the ProxyPassReverse rewrites Location: headers. If you are using Django, an HttpResponseRedirect() function call will usually set the Location: header based on the X-Forwarded-Host but because if you are not using mod_wsgi(), the header may not be correctly set. The ProxyPassReverse provides a way to rewrite redirection requests for this reason.
The other thing is to make sure that your base URL is also set with this /auth/ path:
// Base url to protect
GApps_Session::$BASE_URL = 'https://myhost.com/auth/';