If your evaporation rate is relatively stable, you could rig something with a digital timer. You would have to plumb it so it tees off the main line from your chosen pump, with a valve off each end. Run one to your sump, the other back to the freshwater reservoir, throttle the valve to the sump so it is just open a crack and throttle the other half way open. Don't run the hose to the sump yet. Set the timer for 1 minute, and measure how much water comes through the sump line in that 1 minute time. Figure out how much water you're losing and set the timer to come on for a minute at a time, however many times a day you need (my digital timer has 20 on/off cycles per day). If you need more or less water in a minute, you can adjust the valve accordingly.
Well, that's a cheap and easy way to do it. If your evaporation rate isn't constant, a system like this will need constant dialing in. But it's an idea if all else fails
