Plugin to email digest of changed pages?

Does anyone know of a plugin that will send a daily/weekly email digest of all changed posts/pages/custom types from a site in Multisite?

The use-case is that a client manages 3 sites with similar (but not identical) content, and would like to know when someone has updated Page X on Site 1, so that they can review and see if it's necessary to update Page Y on Site 2. The plugin would just do the email notification - not any actual updates.

This is similar to a "changed blog posts" subscription email, but more generalized & customizable.

Any ideas gratefully accepted!