I know about spectator keyframes, but they're always from the players viewpoint.
I'm on about offsetting the camera angle, so that it follows the same path as the spectated player, but also keeps that player frames the same.
Imagine for example playing in either F5 mode or reverse F5 mode
Use Spectator Keyframes in Version 1.0.6, and double click them to open the "Edit Keyframe" GUI. You'll find a button to toggle the Keyframe type, which allows you to do exactly what you're asking for.