Black Oil Sunflower seeds are often recommended as a supplement (in small doses) for getting a rabbit's condition up or maintaining the condition of nursing does. If I remember correctly, this is due to the high level of fats in sunflower seeds, but I'm not sure if all sunflower seeds are equal in that regard.
I think they'd probably be safe enough as a treat, but too much could get your rabbits fat (which causes problems when you're trying to breed them later).