We're actually looking at adding something similar to this to audiences - so you can assign a role to all the members of an audience and have the role assignment auto-update as people join/leave the audience.

One issue is that roles are assigned to users but also to contexts - so when assigning a role to an audience you would need to specify a context level (e.g. category) and a context instance (e.g. a particular category).

One of the big issues with this will be scalability/performance, since role assignments can be quite slow it will need to be done with scaling in mind or it won't work for large sites.

Simon