Afaik the main reason for having this separate bus is to enable accessibility for administrative applications running as root.  So I _think_ it would work to allow root and the current user to connect but not other users.  This would require that the accessibility instance of dbus-daemon die with the session, since otherwise it stop working when, for instance, the gdm session ends and a user logs in, or a different user logs in.  I am not sure if this is happening right now--need to test and fix if necessary.

Note for the session bus, we don't currently explicitly allow root, though on Linux, it's possible for a process with CAP_SYS_ADMIN (not necessarily root/uid 0) to fake its credentials for the socket uid and connect.

I've modified the .conf file so that only root can connect, rather than all users being able to connect.