hmmmmm.... I have not heard of a card for this, but the GAC from Guest Services may be what you need at WDW... this is a card that is used to alert CMs when a person needs special assistance or accomodation, which you get at the main gate of each park. Haven't heard of using it for diets. Click on the link in my signature for information abou the GAC. The sit-down restaurants area always willing to accomodate special diets with advance notice, and I wonder if reduced portions would qualify as that??? I suppose they would. You might need a brief doctor's note, but only for backup.
Special Reservations may have the answer for you... if you call CRO they will transfer you to the number.