Hi Dominique,
I am the maintainer of https://moodle.org/plugins/availability_role.
This plugin currently can't handle guest enrolment yet (as the user is not really enrolled into the course with a role), but a colleague of me is already working on this. Due to other priorities, it will take 2-3 weeks until it's finished. Watch out for a new version in the plugin repo if you are interested.
Thanks,
Alex