Incorporating fruit powder into a cookie recipe?

I've used freeze dried strawberries and raspberries, blitzed into a powder, in frosting for cupcakes. Might work for cookies, with the powders you have. Have not added to batters or doughs, so no help there.

I like to use those powders when I want to flavor frosting or fondant.

When I use ginger powder, I usually have to use it WITH fresh and candied ginger to really get a ginger flavor. So I understand what you mean by not wasting them on your first try. Good luck!

I have used dried mango powder in muffins, and that works. I used a suggestion to replace 1/4 of the flour with the powder, which worked out to about 1/2 cup of flour for my recipe. I also used it to add to a curry mayo I made for a luncheon chicken salad, kind of like Coronation Chicken inspired, and added it to buttercream to frost a plain vanilla cake. In that case I think I added about 1/3 cup of the powder for a two layer cake frosting. My daughter likes it on yogurt, and in smoothies and milkshakes which are pretty similar tastewise to a mango lassi. I'd imagine the elderberry powder would work much the same way. The only time I used anything close was using a little bit of elderberry drink to cook apples in. Elderberry flavor can overwhelm delicate fruits, and it's generally pretty tart stuff on it's own. But like the mango, I bet it would go well in a buttercream frosting for a plain cake, or as a sprinkle over yogurt, ice cream, or something like that.

Not sure how they would work in a cookie (strength of taste, change under heat). But I've used similar powders and dried fruits mostly as a garnish or mix-in for yogurt, smoothies, bowl of cereal, ice cream.
Also nice when mixed with confectioners sugar to dust on top of cookies or cake.