Bignasty,
you have to consider the effects of the salt on the concrete tank as well as the salt clogging your leaching fields over a period of time, i have a septic system as well and also wondered about the effect of the saltwater on the benificial bacteria in the tank that helps break down the solids.. fortunatly i have a heavily wood yard and use the saltwater to kill the thorny weeds that encrouch on my lawn, its going to be a pain this winter. new tanks installed in my area are 1000 gallons....years ago 500 was the norm..HTH