This might not help you in any way but I have several times tried to use my corporate rate and the marriott site says something like "rooms for this code are not available" and still offers the same room type - most often standard in my case- with way higher price than my corporate rate. I have never called and tried my luck thou, just stayed elsewhere. Maybe they only honor corporate rates as long as there are many enough rooms of that type left?

OT: Is it ok to discuss corporate rates and their logic etc at FT? Because I wonder why at some properties my corporate rate is even higher than standard rate, and sometimes it's so much lower that I can hardly believe it.