cache sweeper not getting called

Yep, I've got the sweeper being used in a couple of controllers - all
of them initialize it, but no model changes are being observed (or at
least the sweeper isn't being alerted to the model changes)

Did you tell config/environment.rb where and how to load the sweepers? That one bite me a while back. If you didn’t add it to config.load_paths in the Rails::Initializer block there. Hope that helps.