While there are a couple of mentions of Bru'n Water in the Water book, its not an endorsement. AJ DeLange and I were the technical editors for the book and that is a reason why you see some familiar references in there. The Z alkalinity concept was developed by AJ while the book was authored. I haven't really studied his concept, but it is somewhat similar to the concept used in Bru'n Water.
Homebrew stores sell what the wholesalers have in stock. Chalk is one of those staples. Chalk needs a relatively strong acid in order to dissolve it. There is only a teeny bit of strong acid in the mash and only a teeny bit of the chalk can be dissolved to provide the pH-increasing buffering. There is more of the stronger acids in finished beer and wine and chalk does have a use in them. But, forget chalk for mashing!
While calcium and potassium hydroxides can burn if you get them into your orifices or onto wet skin, they are less likely to be a serious hazard when handled carefully. This hazard shouldn't be a reason that they aren't carried in the homebrew shops...its because the wholesalers don't provide it.
I've finally come to the conclusion that baking soda is really the best alternative for adding alkalinity to mashing water. The amount of sodium added to ONLY the mashing water is eventually diluted by the sparging water and the net sodium content is not offensive in darker styles. In fact, you'll see that John Palmer did a number of taste tests to prove that sodium isn't really a problem in brewing at modest content. I find that the amount of sodium added for my typical dark beers brewed with RO water is well below the limits suggested by John. The other good thing is that baking soda is stable and doesn't degrade like calcium or potassium hydroxides do. Baking soda is really a win-win.