The best solution is to define a new role (or roles) to cover this situation - which has the advantage that you can control exactly what permissions this role has.
You probably want to start by copying the teacher role, but then change:
- name, shortname and description
- moodle/course:view permission to Allow.
- Context types where this role may be assigned to Category and Course
Then you can add people to the course (via other users) or the cateogry in this new role.
The 'hidden role assignments' thing in Moodle 1.9 was always a bit of a work-around for the fact that Moodle did not really support this way of working, and the new options in Moodle 2.x are meant to be an improvement. I think they are, anthough they may be a bit more complex to understand at first.
Thanks for the quick reply. I've started playing with it.