I'm no expert in this area, but I'm going to give answering your question a shot. I don't think using a salt substitute will get rid of salty tasting ejaculate. A lot of foods have a high sodium/salt content, especially fast food and I'm thinking that it will still taste salty anyway. However, cutting down on salt and sodium is generally good for your health so I'm not going to tell you not to give it up.
Your ejaculate has a lot to do with what kind of foods you eat. If you generally fill your diet with fruits and healthy foods on a regular basis, you'll have a better taste. But also remember that your taste, along with your partner's taste buds will have an affect on your overall taste too. So it's not just what you eat, but what your partner is eating and drinking too and how ALL of it mixes together.
I hope that clears things up a bit for you.
Ann