Several of us have tried this recently and it's worked out well.

Just book a room in the ordinary manner. Then, 30 days in advance of the stay, cancel the booking and immediately make an awards booking. This works for the preferred room, as in the case of a king bed, when those are otherwise sold out.

To be really safe, reserve 2 rooms, and then cancel 1, until you have the award booking, and then cancel the 2nd. Typically the chances that somebody else will be trying to book the same room you have just cancelled is probably remote.