Why would you want to install such a device? The only thing I will do with a Zinsco panel is replace it. I have gotten into hot water with a couple of employers over my policy, but it would be on me if the house burned down. In both Zinsco cases, the employer came around to my way of thinking. When I was told to upgrade Zinsco panels, I demanded an official letter, from my employer, relieving me of responsibility for the panels I might have upgraded. They were pissed off at the time, but got over it. I have had other employers who refuse to work on Zinsco garbage. The replacement breakers available today are Chinese junk.
When told to do something I consider questionable, I rarely bite the hand that feeds. I have replaced too many burnt up Zinsco panels to deal with them.
It sounds like you have an opportunity to sell your customer a service upgrade.