Hi Rich
The short answer is that audiences and tags are separate in Totara 12. Restricting Course visibility by audience is possible so goes some way to your requirements.
Totara 13 uses a recommendation engine to process user metadata, content and interactions to recommend courses to users
I think that the use of tags per audience is a good idea and is what the tag collection idea is about but tag collections aren't currently able to be used separatel by the catalogue.
Regards