How to automatically add contacts to a group that matches the ACL?
As I didn't hear from anyone, I figured I'd just write the extension.
Artful Robot's AutoGroup Extension is here for your delight and delectation.
From the README:

This extension allows you to choose a set of groups which will be added to new contacts if the logged in contact is also in that group.
e.g. If Staff member Wilma is in the group 'Region: Birmingham' and Wilma adds a new contact, it can automatically add the new contact into 'Region: Birmingham'
group.