Turn on light when camera active
Oversight is a product by Objective-See that performs low-level detection of when the camera is used. I posted a screenshot from a talk by Patrick Wardle that discusses this specifically, along with some sample code of how to invoke this via AVFoundation notifications. The portion most relevant to you starts at 43 minutes.

If this application you're describing is for personal use, then I think it's fine, but note that the type of callback you're looking for is that the same one that malicious tools can use to also record your Facetime/Skype video and upload it to a third party. So, please note and understand the full context of what you're proposing if you're not already aware of it.