Real life helicopters have been equipped with a system called Integrated Helmet and Display Sight System, IHADSS, since at least the mid-80s, maybe longer (or something like it). The principle at work in such a system is that the pilot or driver of a vehicle can operate a turret with his head while wearing a special helmet with sensors that dictate the position of the turret. This allows the pilot/driver to stay on course while aiming in another direction by simply looking in that direction (with his head). It also allows the co-pilot to and pilot to switch targeting roles, or the pilot can cover his own escape route if the co-pilot dies.

Real life applications include, but not limited to:
AH-1*
AH-64
RAH-66

Also, since customization is a huge part of why people buy ArmA3, this would enable realistic applications of experimental aircraft, or even hypothetical research on use of fictional vehicles. Examples:

Blue Thunder - a fictional police chopper from a movie of the same name, which had this feature controlling its machine gun.

Cobra FANG - from the GI JOE franchise, a one-man chopper with a front machine gun turret orb (I also happen to be developing a chopper inspired by the FANG in the engine, thus prompting my research and request on the topic)

Airwolf - a vigilante helicopter from a TV show of the same name. I think there was an iteration of it which had a turret.

One-man mini-tanks or walkers that have semi-rotational turrets.

NOTES on USE and OVERUSE
I realize that the use of a pilot-controlled turret system could allow novice developers to make uber vehicles, but ultimately it is down to savvy server operators to limit the availability of these vehicles. There would need to be a special helmet in order to control the turret, so making that helmet a hard-to-acquire item could help reduce its overuse. Controlling the turret in-game should be bind-able to TrackIR, Freelook, Joysticks, Keys, and Mouse - however the user desires - to allow the gamer to have the same sense of control over the entire craft as the pilot would.