Since when are you concerned about Echeverias Tim?
No seriously, excellent point, and perhaps the word "chalky" is a tad misleading. All I can point towards is a positive correlation based on my experience growing the farinose types. There could be some other biological cost which the calcium pays for. Interestingly, I did a trial with E. nodulosa - which is about as chalky as a watermelon - where gypsum was used over the control. The gypsum sample demonstrated much accelerated growth (mostly roots) over the control which had typical root rot. What nodulosa has in common with many farinose types is the relatively high susceptibility to root rot. Perhaps the calcium helps in suppressing certain soil-borne pathogens.
Yes, the cuticle is comprised of mostly long chain polymers(hydrocarbons). I think in the case of farina, they form flakey scales. If calcium is actually present, the only compound I can think of would be crystalized calcium oxalate, but isn't that poisonous?