I think this is a brilliant idea but I have a few suggestions. I would use a closed bag with an over pressurization valve. I would have a tank that can carry enough air to fill the bag attached to the bottom of the bag instead of the reel. This would reduce the size of your handle making it easier to store in a pocket. The tank hanging off the bottom of the bag would act as an anchor of sorts to keep the bag upright at the surface. With a closed bag and a tank that has slightly more air then needed the bag would be fully inflated and sitting upright at the surface. I would also use a simple fitting that can be purchased at any dive shop to refill the tank from a BCD hose.
Tire Inflator at LeisurePro Doing it this way you now have two products to sell, a bag with a tank that will fill it to the exact amount needed that could be used with any reel and a self rewinding reel. Cost is always a consideration but if you can simplify this process enough and give them a two tier option when buying there are a lot of people who will have no problem with the price.