Why? GearVR gets ~2 hours of usage that's more than enough for most VR play sessions (it's actually quite tiring).
And GearVR also has to run Android and the rest of the cell phone crap and the cell phone form factor limits the battery size.
You likely going to be easily be able to get 3-4 if not more hours of playtime from a pretty light battery by using just a slightly bigger battery than in an high end cell phone and while optimizing the hardware for VR rather than general purpose cell phone usage.
Wearing a power pack on your back with a cord to the head would be quite fine. The only thing to be careful about is batteries that get too hot. But in a weird way, that's better on your back because I think if people were forced to choose, a fire on one's back is better than a fire on one's head. No fire is of course the best. :)