When I was flying for AA, the crews could not request special meals. But my husband who flies for DL can, then can request things like low-fat, or veggie.
We had pretty stringent requirements for crew meals to be scheduled for a flight. The flight had to be over a certain block time (2 hours if I remember right) also there had to be more than 3 legs with no ground time at any station to get food ourselves. Basically the company said that if we had a schedule for the day such that we had more than an hour on the ground at any one station, we were on our own to get off the airplane and go scavanging for our own food.
Our crew meals consist of Subway sandwiches. One of our pilots is a vegetarian (actually vegan (sp?)), and also alergic to flour. So she just brings a mini-bag cooler of food for herself on trips, and that seems to work out great. I'm sure if she requested a vegetarian meal, the company would probably provide though.