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.
I've thought about it, but it would be very difficult and expensive. You would need insane filtration and good protection from rain
