Comment 32 for bug 857651

What about creating a system group like "noguilogin" that accountsservice uses to determine whether a gui login is allowed and passes that as a flag via dbus with other requested user information. Then all other services that use accountsservice can refer to this when validating whether an X session should be allowed. Also, services that don't utilize accountsservice can just look for membership in the group.

Another option may be as simple as changing lightdm so there's a way to disable using accountsservice. Then you could edit users.conf as you normally would. Not pretty, but it gets the job done.