How to boil sushi rice for yakimeshi?
Typically bouillon cubes include a pretty hefty dose of salt, so you're probably fine without adding more. You can always salt the finished fried rice if it needs more seasoning.

All your calculations look fine to me, but one thing you might need to consider is that the salt would raise the boiling point of the broth and potentially affect how the rice cooks.  Probably not a big deal, but if the recipes you've found are particularly insistent that the rice be cooked only with water, that might be why.  Just hedge any cooking times a little and check the rice as it cooks, and you'll be fine.