[jedberg]

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

[js2]

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.

[mckirk]

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