The scariest water I have ever seen in my life!

[waterbear]

Although you've got the high pH of the Sodium hydroxide, you also have the low pH of chlorine gas (which instantaneously converts to 100% pure sodium hypochlorite - without the high pH nature) to keep it fairly neutral.
We typically say that if you just leave the pH alone for awhile, it will climb to about 7.8 - 8.0 and hover around there, and not much higher.
Yep, my pH goes up to about 8.0 and stays if I don't add any acid.
The Unichlor systems of old (the one you're describing with the waste Lye tank), were modified in the late 80's to reintroduce the lye as a pH balancer. They called this the diluter kit. Again, one side produced chlorine gas that was syphoned into the return line, whereas the caustic side was commonly drained, but then was used to return some sodium hydroxide back into the water, thereby increasing the pH back up from the low pH effects of the chlorine gas.

thanks for the info!