My initial thoughts were that I could use a syringe after a print to suck up leftover resin and put it back in the resin container. When I get close to the resin/water boundary layer, use the syringe to suck up resin and some water and place into a small temporary container. It will separate over time and I can extract a little more resin out of that. Also, take the bottom printing container with salt water in it, and swap it with the top container.

There will be evaporation over time and that will increase the specific gravity of the water because the salt doesn't evaporate. To restore the specific gravity, just add pure water to marked fill lines. And after a print, place lids over containers to minimize evaporation.