This change requires custom work and can’t be done with a plugin, not that I am aware of, since the “email” system has to check all new properties, compare user preference with property settings, then send email. So it’s a more complex work to do.
What we wish to do for the future release is save search alert (so it’s not at registration), but it’s still an alert system. How it will work, I don’t know yet. It’s the kind of feature we can know for sure how it works when we finish it, because many details can change during coding process.
If your client can wait a couple of weeks (my projection is roughly estimated), then maybe this will be a good alternative to your description?
If you need something else, we can review your custom development projects, but we’d still be available to help in a few weeks based on the schedule we have ahead.