How can I correct dead Macbook Air camera sensor pixels BEFORE other applications use it?
I'm not sure what mechanism you would use to inject your code in to the firmware of the camera and affect the data that the system sends directly to another app. Similarly, modifying the OS would be tricky.

For the time and engineering you would put up, it would be far cheaper and quick to just add a USB camera and have a good signal in the first place. Depending on where you ran the code, you might even save power / battery by having a good camera send good data rather than process real time video corrections.

That being said, you might be able to use a VR processing type software to send a video stream to an app that's ready to receive it. I don't think you'll have luck with FaceTime or Skype / Teams unless they have a plug in / filter that already does the processing you want before sending the video out.