Firing a hook on smart group status change?
I think we have a client where a postcode look up (on Contact save) triggers creation of a Relationship being made. The Relationship connects the Individual to the Regional Group (contact of type Org'n). You could then use our Relationships as ACL extension to run your 'access rules' from. I will ping JoAnne to comment further if she can.

From JoAnne:  I'll add to this rather than comment.

There is a table in the db linking every postcode/suburb combination to one of the 200+ organisations of type "local group".  

The initial relationship is created when a person signs up for a specified membership type. 

With any change in a member's primary address, the relationship is re-checked. If warranted, that relationship is ended and a new one created with the new local group.