Dissolving/denaturing bitterant residue
Since probably only ppm levels of denatonium benzoate are in the compressed air, there are ppm levels or lower of denatonium in your flour grinder.  At that concentration, water should dissolve denatonium benzoate.  Is that a viable way to clean your grinder?

According to this solubility table, the best solvent for denatonium benzoate is methanol.  However I would not recommend using methanol on anything you are using for food.  Ethanol is only slightly less good.  Maybe you can wash your grinder in some cheap vodka a few times?

Here is a random thread from Google Groups where someone claims to have identified a vendor for non-bitterized air duster cans.