Says that it's boost-only in all the literature I could see, so it can only add power to the prop not generate with it. Regardless, you don't really get into a position where you're harvesting energy in a plane - you just use less power while you're descending. Unlike in a car, most of a passenger jet's flight time is at speeds where drag (which squares with speed) basically means you'd have to nose down at a very aggressive angle to actually pick up speed without the engines providing thrust. The plane's engines are almost always under some kind of load until it is on the tarmac and slowing down so there isn't any opportunity to "regen" during a normal flight.

I'm not sure how that would help. If you're descending it's probably to land, and then you can recharge the batteries with electricity from the ground. That would be more efficient (and cheaper) than burning fuel to charge them.