It just seems to me that someone is making you use the wrong heater(s) for your application. My latest crazy thought would help if they were being installed in 2's. You could use a dual diode module and drive each element from opposite half cycles. So you cut your power in half per unit and not even add any odd harmonics. A scope would be the only place where you might see a tad of crossover distortion. You could still add phase control upstream.
Joe
