I tried it thinking the same thing.
It won't work unless your SLAM level is low and your SWG produces a bunch of chlorine.
During a SLAM, the chlorine loss during the day is going to be much higher than your typical daily chlorine loss due to the loss being a percentage of TC, not a finite amount like 3 or 4 ppm per day.
It may work at night when there is no sun, but then you lose the ability to perform an OCLT.