I'm also trying to use a Ritz e-certificate for a legacy SPG property. Like roubaolic and others, Marriott is telling me that that functionality is not currently supported, and gives no time estimate. I can see that availability at the property I'm looking at is diminishing.
I'd appreciate it if people would post here their experiences using Marriott certificates at SPG legacy properties. When someone posts that it is now working, we'll all know - thanks.