I tried to load my rewards dollars onto the account at check in, was told I should just give them my credit card (I knew I was going to spend more than I had available in rewards dollars) in the terminal, and could go to Guest Services to pay toward the stateroom bill w/ rewards dollars. And the CM at the desk had no idea, but the senior/supervising CM showed her how to do it.
I also used the rewards dollars in advance, right through DU's website, to make interim payments in between deposit and when final payment was due.