I agree with this analysis. If HHonors allows the booking, either on its web site or via a Hilton agent, then the booking is not a "mistake."
Furthermore, there are HHonors reps monitoring this site. If booking past 12 months is not allowed, the reps should speak up and plug the booking "glitch."
It would be unethical on HHonors' part to say/do nothing until after March 27 and then announce that the bookings are invalid. It would be completely devious on HHonors' part to allow customers to travel halfway around the world to find out that their reservations won't be honored.
HHonors has pissed off enough customers that I don't think it would chose to alienate us even further. Just my two cents.
I don't see how they could refuse to honor it. You have a confirmed reservation which was made in good faith. The fact that it is +12 months away is immaterial.