It probably depends on the restaurant and the CM who happens to be helping you.
At Columbia Harbor House, we were able to substitute a cup of chowder for the dessert (this was a couple of years ago, but it was the CM herself who suggested it to a man in front of me who didn't want the dessert).
At the Pop Century food court last month, there was a huge list of things you could substitute for the dessert-salad, celery & carrot sticks, fruit bowl, cheese nachos, etc. We substituted frequently when dining at our resort.