I personally am not sure if it would be toxic. I know the berries can be (like a previous poster mentioned). I have a link somewhere with safe plants but it was posted on another forum. I can PM it to you when I find it.
BDs will eat a wide variety of foods. Usually when younger they will eat more bugs and similar creatures than as they grow, when fully grown they will prefer to eat leafy green foods, but the occasional meat is needed. They eat a huge variety of greens so it is easier to know what they can't eat, which is the following:
- Avocado
- Rhubarb
- Spinach
- Kale
- Cabbage
- Citrus fruits
So elder flowers won't poison or kill your beardie. I think they are safe.
There is a very long list of what they can eat and what not here:
http://www.reptilepets.org/keeping-bearded-dragons-as-petsKale and cabbage are fine to feed in moderation. Avocado is toxic, as are onions and garlic.