Just for cutscene use, a controls-off sexp would be great. ATM the best one can do is disable all ship subsystems but this makes a loud noise when one's subsystems explode. And giving control of the player's ship to the AI in conjunction with a play-dead sexp doesn't actually prevent the player from firing weapons or moving, just pointing the ship somewhere. In any case they can reactivate the HUD by pressing Shift-O.
The most concrete way to prevent the player from doing anything is by disabling the controls, perhaps except for going to the options or quit screen. Would this be do-able?