Prepay?
I would be surprised if you were ever allowed to prepay because the future year's fees have not been assessed yet.
For Hilton, using your points early means that you will pay the 2017 price (almost always more expensive because of inflation and based on expected 2017 costs) and Hilton only incurs the 2016 cost. It is a windfall for Hilton, but a nice benefit for users. It seems like Hilton realizes this, which is why it does not charge for borrowing, but does charge for just about everything else.