The next question is where would you get food grade lye from? I know I can get food grade lime as pickling lime.
Martin, as someone said you can get food grade lye online -- specifically at sites that sell soap making supplies.
I have some that I bought to make laugen brotchen and Swabian bretzels (pretzels); the lye is added to water and you dunk the risen rolls of bretzels into it just prior to baking.
On adding chalk: something I tried once was to crush my dark grains serpeate from the rest. I pladed them in a plastic bag and added the chalk, then shook together to coat the crushed grains. All the chalk seemed to stick to the grain bits...this was then added to the rest of the grains. My thinking was that the chalk would be right at the site where the dark grains would release their acidity,and so aid getting the chalk into solution.
So...does this sound silly? I have no means of testing if it makes any difference...no pH strips, no pH meter.