raise priority of the sso_login and sso_logout handlers
It shouldn't be necessary to exclude /sso_login and /sso_logout from the serving handlers (as done in the Apache config snippet in the documentation with the ProxyPass "!" directives).
It's possible that this could be done by setting somehow the priority of the installed content handlers to something higher than mod_proxy (I haven't tested what happens in a standard setup if a file named "sso_login" is already present).