I suspect that the accuracy of such devices likely won't be able to outmatch the built-in mechanisms your body uses to optimize hydration (i.e., thirst). Sodium is a little trickier, but honestly, most people dont need to worry about sodium until you get to full day events, and in that case, you also have cravings for salt when you need it.
Best thing that I think you can do is weigh yourself naked with any flasks/food before a run, and with any empty/partially consumed flasks/food after a run. Just make sure to dry your body off as well as possible. It's probably only worthwhile on your longest runs where you might lose a couple kilos of sweat. Theoretically, you also exhale mass of co2 from whatever calories you burn, but that should be minimal compared to your sweat.