For those that say that chalk works in a mash, I'd ask: what's your proof? You think it works...or do you have actual pH measurements with a reliable meter? It will have to be the later, if you want me to believe your result.
I'll answer the second part first. I have an MW-102 which I calibrate periodically and also on the rare occasion that the measurement is far off from the hypothesized value, though I cannot recall recalibration ever dramatically changing the measurement (>0.05).
I have a supporting anecdote for chalk affecting the mash pH. I'm not sure I'd call it "proof", but an observation that I have been able to explain only by chalk having an impact.
I have water with high temporary alkalinity (~80mg/L Ca, 320mg/L HCO3), so I pre-boil it for light beers. At some point I read that adding chalk when boiling adds nucleation sites, and causes more CaCO3 to drop out, so I did that, and indeed the post-boil TDS reading was something like 10% lower. Without added chalk the water clears in maybe 1h, while with added chalk it takes something like 12h (and if someone can explain what the difference is due to, please). Since I often boil the same day, the first time I did it I proceeded anyway without waiting for the water to clear, and decanted the cloudy water when there was the usual layer at the bottom of the kettle. The resulting pH was as expected (yes, yes, keep reading ;-). However, in the next brews doing the same with cloudy water my mash pH was through the roof. After significant head-scratching I pinpointed the differences as a) the first beer was higher gravity than usual b) I switched from acidulated malt to lactic acid after the first. Playing around in Bru'n Water, I recall discovering that acidulated malt is not accounted for as 3% lactic acid, but uses some other formula, which led the high-gravity beer to being acidified proportionally more, and therefore cancelling the chalk still in suspension. In subsequent brews, when I waited for the water to completely clear before decanting, the Bru'n Water predictions started nearing the measured values with lactic acid. So, I concluded that the chalk in suspension does have an impact on pH.
To double-check if my conclusion was correct, I prepared water samples mimicking the cloudy/clear boiled waters as described above, and titrated. Indeed it takes more acid to drop the cloudy one (my notes from a few years ago are probably in some stratum on my desk, and if putting a number on "more" is important, I can try to dig them out).
Now, granted, that's an anecdote about chalk I wanted to remove affecting the mash pH, not added chalk affecting it. I am *not* implying that other observations from adding chalk are inaccurate. The only difference I can imagine is the boiling. Maybe the boiling physically "unclumps" the chalk so that it's extremely fine-grained in suspension? (completely thinking out loud there)
In any case, everyone [with a pH meter] can try titration to see if chalk has an effect. If it doesn't, try boiling the water with the chalk to see if it makes a difference -- I did not test adding chalk to water without boiling the water, as I was working in the confines of what was relevant to me. If even that doesn't make an observable difference, and there is no water with high temporary alkalinity available, one could dissolve chalk into carbonated water as you suggested, and boil that, and repeat the titration, to minimize the different variables in the experiments.
Also, in the riddle-me-this department, if undissolved chalk doesn't affect the pH, why decant at all after boiling? Every text I've come across for removing temporary alkalinity by boiling is boil-wait-decant.
Finally, on a mildly related tangent, I also tried gypsum for nucleation sites to drop CaCO3 out, since gypsum is supposed to have poor solubility at high temperatures, but it did not drop the TDS reading -- accounting for the extra TDS from gypsum -- so I guess it's "soluble enough".
