I respect your position, but in this case it's only authentic in the sense it exactly matches the municipal profile that Rochefort uses, which at the very least is treated with a good amount of acid (1 liter of Phosphoric in fact, see attached Rochefort brewing sheet) to reduce alkalinity of the mash water.
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By all means, please pursue what you want to. Pickling Lime is the best way to achieve what you are after IMO.
Just know that adding alkalinity to then turn around and reduce it, just to get elevated levels of Calcium without Sulfate and Chloride, won't net you much.
It certainly won't increase the "Rochefortey-ness" of the resulting beer. That's purely a product of their fermentation practices and high carbonation.