i think that one way to mitigate the inefficiency of using salt this way is to keep another tank off the "waste" water that's constantly coming off the main tank. tinytool mentioned keeping a nice frag tank this way (no skimmer, etc.). you know the Ca, dKH, pH, etc will be stable. obviously it would need to be a much smaller tank. but it could either do frag work, work as an ongoing quarantine (if you're in stocking mode), or even use it as a species tank (anemones come to mind but may be too messy).
-tE