You can prebook the Pinnacle -- but you won't get the 50% Loyality discount until after you have eaten there. You pay the full price via credit card when you book. Then after you dine at the Pinnacle -- you get shipboard credit for that 50% Loyality.
Ever since this new program began, every cruise we have to fight to get our 50% credit. On one cruise we were still fighting for a Pinnacle dinner that I had prebooked for a night that was 3 days before the end of the cruise. Some people have no problems getting the discount. Don't know why we have so many problems.
HAL could and should develop a program where it recognizes that we (meaning all people) are 4 star Mariners and give us the discount when we prebook.