I'm not sure that Ospreys are on the list of potential falconry birds in most states anyway.

I have had several birds that would score on prey in the air and either come back to the glove with it (Merlins) or back to the roof or shell of the truck. Most of them did this naturally without much attempt to enhance it.

But a bird that carries from the ground and away from you, would be a much different scenario as there is more functioning in that reaction than just the act of carrying, i.e. greed, fear of you stealing the prey, distrust, fear of eating on the ground, etc.. These issues can often be difficult to overcome just to be able to secure the hawk/falcon with their food.
Once you start catching game with your bird, I'm sure that the least of your worries will be how to get it to retrieve. It will be more important to focus on where the next best slip is going to come from, which will keep you busy enough I'm sure.