On the regular plan, no, not under any circumstances, including medical conditions. This is one of the dining plan rules that is consistently enforced. There are also the considerations that desserts are cheaper and easier to produce than appetizers, and Disney has in effect hastened the turnaround time of tables on the regular dining plan by ensuring that half the party won't order appetizers, the other half desserts to share, unless they pay extra cash for the appetizers.
You can request an alternative dessert, possibly fresh fruit or a cheese plate or something to that effect, but you cannot substitute an appetizer.