You're mistaken. The screen reflects a wider area than the screen itself is. Someone standing off to your side with a light in the hand will be clearly visible.
Sorry if I say it but I am not mistaken.
I won't be able to see the lightbar reflected in the screen and that's s because I actually verified it.
I took my iPad, tun it on, put a full blue picture on in, held it a the same level I hold the controller and pointed the screen towards my Tv and I could not see the iPad screen reflected into the screen.
I don't game like this or like the guys in the Ps4 Palyroom video.
Even my local co-op buddy DS4 won't be visible reflected in the screen unless he stands up and plays that way all the time and that's not going to happen.
That's basically pointless though. Exactly the same result can be achieved by taking a glow texture and applying that on the screen. Using the lightbar as a reflected light showing on the screen adds nothing to what a game can render to the screen directly.
Well they are using a visual cue to show when you are it the darkness similar to the one used by Riddick games but the lightbar is there to give you the impression you are holding the lightgem itself in you hands winch is a nice touch in my opinion.
The lightgem is also displayed on the HUD but it's
this sphere in the bottom left corner and its very intrusive and adjusting the brightness of the lightbar is in my opinion a better idea than use HUD.
Now I personally would not use the lightbar as alternative to HUD.
I would first design some neat visual and audio cues, then use the haptic feedback and as a last resource use the lightbar.
I would use the ligthtbar to enhance engagement though.
To make an example in an RPG I would make the lightbar glows when you get closer to a secret area or in proximity to certain enemies a la Bilbo's Sting.
Sorry for the long reply anyway.