Oh yeah,I'm no expert. I can see salt being problematic if the system sprung leaks and contaminated the soil which wouldnt be uncommon once you have tens of thousands of houses rigged up. Im pretty sure most water based systems just use water and antifreeze.

Point is that the fundamentals are simple, when theres excess electricity and nobody is home convert it into stored thermal energy that can be used later when people are home, the devils will be in the details.