The public aquariums do it..
but yeah, the collection points are way out there.
Down in the L.A. area I noticed the LFS's were selling a product called "Catalina water." Same idea, just huge vats of it so you could buy 40 gallons at a time if you wanted. They take a boat out towards Catalina Island, suck up huge amounts of water, take it back to the LFS's for selling. I never really understood what the benefit was though, for the cost of salt compared to the cost of anything else for the hobby, it seems like a lot of work. (This was ten years ago though that I was down there, maybe they don't do it any longer.)
When I was reading up on Balling method (the "true" Balling method, not the 2-part/3-part automated dosing we tend to do in North America that we just call Balling), the added salts put into the aquarium cause the SG to rise and so water needs to be taken out of the tank and replaced with RO/DI to maintain a steady SG over time. All done with slow peristaltic pumps so it's a more or less steady state situation. Anyhow I sort of wonder how this compares to a "continuous water change" too.
Kien have you considered Zeo a go at all?