There's an important distinction to be made: Android in its stock form, and Samsung's version of Android, with the Touchwiz framework.
The Touchwiz functionality for the eyeball recognition works with only with apps 'native' to Samsungs Touchwiz version of Android. Apps like Samsung SMS, Samsung Internet, Samsung Gallery etc.
Air Gestures can be ported to 3rd party apps, Flipboard is an example of this.
Fore the eye recognitions stuff, like smart scroll etc. the app needs to be embedded in Touchwiz. However, the 'keep screen on' functionality works over apps, so the screen shouldn't lock while gaming etc.
After looking into this myself, I havn't found any Xposed or other modules to override this for root users. Also, on the S4 Google Edition (running almost1 stock Android) there is no eyeball recognition whatsoever, and cannot be implemented.
Also, from a developer's point of view it, even if it was possible to access this functionality, would it be worth implementing it for only one device, and not being accessible on any other phone? It's a good idea in a way, but I don't see it expanding for a while, until other OEMs decide on similar tech perhaps.
1 Techincally the Touchwiz framework is there to enable the S-View cover. On the HTC One Google Edition HTC Sense is there to allow Beats Audio to be built in.