Hello,
I would not reccomend it, as dehydrating, as well as freezing vegetables can destroy alot of the vitamins and minerals in them.
There are ways that you can dehydrate to reduce the loss of some of the nutrients, such as blanching the vegetables first. You will still lose some of the vitamins and minerals, but that would also help to retain them during storage and rehydration. The problem with that is you will lost all of the water-soluable nutrients.
Rehydration can also destroy some of the remaining nutrients.
I understand how much easier it would be, as we dehydrate alot of foods and herbs from our garden, however nutritionally your dragon would probably end up getting something nutritionally equivalent to lettuce with trace amounts of the nutrients that were once in the food.
Sorry.