Is it possible to capture the output of the AVPlayer using AVCaptureSession? I believe it's possible but can't figure out how to go about using the AVPlayer as an input.
You cannot plug an AVPlayer into an AVCaptureSession, although you can get access to the player's video and audio in the form of CVPixelBuffers and AudioBufferLists.
This is achieved via two APIs: AVPlayerItemVideoOutput for video and MTAudioProcessingTap for audio.
Despite being a c-api, MTAudioProcessingTap is easier to integrate as just like AVCaptureSession, it pushes you samples via a callback, while with AVPlayerItemVideoOutput you pull frames for a given time.
For this reason, if you want a AVCaptureSession-like experience (real-time, push), you should probably let the audio tap drive your frame-pulling.
There is some AVPlayerItemVideoOutput sample code in objective-c here and in swift here and an example of using an MTAudioProcessingTap in swift here.