You might want to consider doing it yourself with an Apex (or other controller) and a Salinity probe. The main problem I would see is working it in with the ATO and maintaining a constant salinity. Between the ATO, Skimmer, Outflow, Inflow, evaporation, salt creep, and any dosing keeping the salinity in check may be a challenge. The weekly water change gives you a chance to manually adjust the salinity up or down slightly (usually up a little).
With the Apex you could dump X amount of gallons per day down the drain and then allow the ATO and salinity probe with a salt water reservoir to handle the replacement water and salinity up/down. I am sure there is some code already written to handle something like this, but if there isn't the code wouldn't be too complex.
Make sure you have a lab-grade probe. I have a pinpoint consumer level probe and it is a HUGE pain in the butt because it doesn't stay calibrated for a long time. The regular salinity probes aren't like pH probes where they are fine (or at least very close) for a really long time - I would hope the Lab-Grade probe would be a little better.
You could do it really simple with the 2 pumps like suggested above, but I would always be worried about timer inaccuracies, pumps not being consistent, something getting stuck, etc. And you could have to check salinity constantly to make sure it isn't way off.
