I was getting a similar experience with white tongue in the very back of my mouth. I can say that doing oil pulling/swishing with sunflower oil with about 3 drops of oregano oil has cleared my tongue. I can not say that this is permanent, as I am in the middle of a candida cleanse, and am not certain that everything will be in balance long term, but I can say, that my results were fast and so far lasting with continued OP with oregano oil in the morning.
Check out the Oil Pulling/Swishing forum for details.