The only time one could apply decoupled head rotation is after the game applied the player rotation. The catch is that for performance reasons games remove invisible objects before rendering. So if you you apply independent head rotation afterwards, you most of the time end up looking at, well, nothing. There are a few ways one can think of to at least partly get around this, but none of them works good enough, this was tested A LOT in multiple ways.
Long story, short: Except the few cases where a game engine supports this (like the ARMA games), there is no way to make this work in a decent way without changing the game’s source code.