Sorry I'm late to the conversation as a new PM+ customer.
The Windows SDK has a function called ReadDirectoryChangesW that tracks changes to a folder and its contents... I don't know if MACs have an equiovalent as I don't program them.
The Windows function is difficult to work with and what it reports requires interpretation, but it removes the need to scan folders for changes. You 'just' start a thread that registers an interest in a folder and it waits until a change occurs. For image collections that have one primary folder, with subfolders containing the photos and defining the structure of the image collection, then registering an interest in the primary folder is enough.
A few years ago, I wrote a C++ app to monitor folders for changes to my images and, after a lot of trial and error, found this function to be the best way to do it in Windows. If the Mac SDK has a similar function then it might make this task doable for PM+.
From a Digital Asset Management perspective, automating things is desirable as it's easy to forget to add new and edit images to the catalogue, and re-scanning/syncing can be slow...
Andy