I stand to be corrected, but I think you'll be just fine. I've made reservations like this all the time. Never a problem.

When I've made these over the phone, I've made the comment that I don't have the points now, but I will at the time of my stay. Nobody has ever said anything about this being not allowed.

This is not to be confused with the airlines, where essentially you typically do actually have to have the miles before making a reservations using a mileage reward.