The byte data includes the hand positions, and all the API data of that frame. The ability to record and playback is already in the Hand Controller! Examples can be found at:
Basic Demo Record Scene:
Basic Demo Script to Record/Save:
After you've recorded, you can enable the Record/Playback as shown above, and copy the recorded file to the Resources folder and select it in the Recording File field.
To play it manually, you just need to call handController.PlayRecording() and it'll start playing the recorded file based on the Graphics and Physics model in the Hand Controller.
Hope this helps! If you're stuck anywhere, let me know.