From my limited experience, if you cancel an award night, it will go back to the inventory right away. A few years ago, I tried the following, and it worked. (Only try this if you are willing to loose this reservation, and book elsewhere)
​​​​​​1. Call Hilton reservation/diamond desk. Ask the rep to search for the award night. This will return no availability.
2. Tell him/her to be ready to refresh the search.
3. On your end, cancel the reservation. Then ask the rep to search again.
4. The room most likely be available to book again (in my case, it did).
5. Once the rep grabs the room, he/she can attach the cert to it.

I also had an award reservation at the Grand Wailea and had to cancel. The room went back to inventory right after I cancelled and was available for booking, even though there were no availability before that.
Try it at your own risk 😁.