I suppose you could. You need an ADR and will need to put a credid card down to hold your ressie. Once you arrive and are seated I don't see why you couldn't order just desert, personally I would feel funny doing this b/c it is the most upscale Disney resturant I've eaten at but that would be up to you. Also remember they have a "resort casual" dress code, i.e. no shorts, sandals, tees, tanks, etc...
We had dinner there during Wishes and also had a window table, we steped out side during the fireworks, pretty nice!