OK, here goes.
1. At the high pH of saltwater, the calcium will precipitate out. Not only won't you get any Calcium, you will also reduce your alkalinity and end up with a scale problem as well.
2. Kalk only has about a 3% solubility in freshwater (RO). Even if you didn't have a precipitation problem, your solubility in a higher TDS water like seawater would be very low.
Nice idea, but there are some small problems with the chemistry there. Just so you see what I mean, try taking a small container of tank water and dropping a little Kalk into it. You'll see what the problem is right away. My guess is that your Kalk reactor would be scaled up solid within a day.
Gary
125 SPS, 75 gal. LPS/softie reef, 9 gal. Nano