Backabit,
I completely understand the science behind what you are wanting to do. The amount of salt you want to use will do absolutely no damage to your environment.
Pond Frog is wrong.
It takes more than 4,000 ppm total salts in fresh water to disrupt fish's ability to reproduce. That's more than 10,000 pounds of salt in one acre foot of water. And, even with that much salt, fish and many fresh water plants can still live and thrive. Salt is a critical compound used in transport of fish. It helps the fish keep from sloughing their slime, it fights parasites and stabilizes crowded fish in transport tanks. I managed a hatchery for several years where the water supply naturally has 3,500 ppm salts. People used that water to irrigate coastal bermuda, but they didn't let the water sit long. They flooded, then drained the fields and the grass took up what little salt was left, then the water went on down the irrigation system. My fish never had any of the traditional diseases and thrived in that water.
The home recipe you want to use will not do any damage to your pond. Use it if you wish.
If you use it, mist it onto the grass and a gallon of the stuff will cover a lot of ground.
Salt, vinegar...or even any governmentally approved herbicide...if overused or abused will cause environmental damage.
Bottom line is this...what little bit of salt (1 cup) used around a periphery of a pond will do no harm.
Looking at the picture of your pond, I bet you won't need to use more than a half gallon of that mix to do what you want to do.
