That’s a tough one- regular flour would significantly change the texture of the meringue, because of its ability to form gluten.
I don’t know if this will work to your liking, but if you were determined to experiment with the flour/butter substitution, you could try rubbing the butter into the flour to coat the particles and reduce its ability to form gluten. Rose’s excellent pie crust recipe does this, as does S. Yard’s croissant recipe. I have not tried it myself.
Another thought is to substitute cocoa powder, though to do that for all of the nuts might not work, taste-wise.