Charcoal is just wood burned without oxygen. Charcoal is just ‘activated’ using steam - all it does is increase the surface area to increase absorption capacity.
I can’t see it in the NPA list, nor on Natrue. However, this would be acceptable by Natrue because Charcoal would be considered a natural product.
But if I were to look to the Australian definition of ‘natural’ it would include very few ingredients - even something as simple as Lavender oil would be considered synthetic because steam distillation does not exist in nature.
By all means use a natural standard - but it comes at a cost. Weigh up whether the cost of an external audit is outweighed by the sales of that product meeting that standard. If it does, it may not be worth it.