Of course, I’m not the one perched 75 feet up the mainmast hanging in the breeze, but my advice is: Don’t worry about it. I certainly don’t dispute your point that saltwater is corrosive. But it’s not as if the buckles are going to melt right then and there. A regular visual inspection for cracks in the buckle should suffice; a little checking and discoloring on the surface of the metal is going to have minimal impact on the strength of the buckle itself.
Not that I’d be completely casual about it. I’d rinse my gear in fresh water after each race, as that saltwater (plus sun) is going to weaken the webbing as well. Hang the harness and dry thoroughly in a shady place. And, I’d suggest replacing the harness annually, if it does indeed show sighs of corrosion.
I know that stainless steel webbing buckles are in fact available, so you might also check with a marine hardware store to see if buckles are available to fit the webbing in your harness. You also could back up the harness fairly easily with a chest harness made of one-inch nylon climbing tape. Make a big loop of the stuff, then pass your arms through it so that the loop passes under each armpit and meets mid-chest. Link it there with a stainless steel carabiner. When going aloft, tie a bight into a tail of line, and clip that to the carabiner. The tail then goes to the harness, where your main connection is made, and from there to the rest of the rope.