We will add a Ca probe to our product line when one is available that is accurate, good for continues use, and low maintenance. Currently there isn't a probe that even comes close to meeting those requirements.
Personally, I think that an alkalinity monitor is more useful than a Ca monitor for a reef tank. Alkalinity is the first to be depleted when there is a problem Ca reactor, Kalk reactor, etc. If you have a Ca/buffer replacement system that puts Ca and Alkalinity in at the same ratio as the corals are removing it, there is no reason to continuously monitor Ca. A check every once in a while by a test kit is all that is necessary.
Alkalinity can be indirectly determined by noting the daily pH swing in your tank. On my reef tank if the pH swing is greater than about .20 - .25 units, then the alkalinity is less than 10 Meq. If you are seeing pH swings of .3 to .4 daily then it is quite likely your alkalinity is low, and your Ca/Buffer replenishment system is not keeping up with demand.
Curt