Although now that I think about it, a clapperboard wouldn't help here because the audio was being generated on the Mac. There was no actual microphone involved capturing the sound a clapperboard would make. I need some other way to measure the latency being added by EpocCam. Hmmm....

Your "clapboard" would be a series of notes played on the keyboard in view of the camera.

That's what I did but it was surprisingly difficult to tell exactly when a key is pressed. It's just not as precise visually or aurally as a clapboard.

Maybe film some display on the keyboard that changes visually with a button press? That could work for synchronizing.