You might consider boiling 1/2 lb of maltodextrine in a couple of cups of water to dissolve it, then add that to the fermenter. The yeast won't be able to break it down, but the bugs will and that will give you an acidity bump.
Can lactic acid bacteria break down dextrins? I always thought brett broke down the complex stuff, but lactic acid was always fermented from glucose.
My copy of Wild Brews is packed away, else I'd look when I got home.