Easily remedied by an adequately sized auto top off. Salt creep has been an issue since the dawn of aquaculture.
If I was trying to maintain an outdoor saltwater system, whether it be a large tank or pond, I would want it hooked into a rather large sump located in a temperature controlled substructure like a basement. Depending on your location a powerful heat pump may be a necessity to maintain the tank in colder times, and the pond would need to be sheltered from the elements for sure. For a 500-1000g pond consider a polyethylene tank or similar of the same size as the sump.