I actually used the powder last night to make our biscuits, also just a few days ago. Both times I made the mix the first thing, mixed it just a bit, then let it stand while I put together the rest of my mixes. Right before adding the buttermilk mix, I stirred it again as there were still lumps and they finished dissolving right away so think I'll be doing that from now on.
BTW, I don't see why adding the powder in with the dry ingredients wouldn't work as long as you give it a good mix. You have to do that anyway if you are adding salt, baking soda/powder, etc. Obviously the last 3 ingredients have to get distributed equally or your recipe wouldn't come out right. You could always add it in with sifting, either thru a sifter or by putting thru a wire mesh colander, if you are worried about lumps.