Got the problem solved with a call to reservations today. The agent was able to apply the e-cert without involving Customer Care or anyone else. I guess that HUCA is a useful strategy with Marriott. I've become accustomed to in on United.

Has anybody else had difficulty in attaching e-certs from credit card renewals to a one night room reservation? This worked fine for me over the web last year, but this year not so much.