Good question. I assume that the Day/Night action is accomplished by gluing two wedge shaped sections of glass together, then silvering the back side that will be inside the housing.
Among my other expensive hobbies, of which I consider my ZX Turbo to be one of them, is Amateur Astronomy. I've been interested for years in building my own telescope, including grinding and polishing my own primary mirror. Once upon a time, amateurs would take the final figured and polished mirror, and apply a silver layer to it. The process was long and difficult, involving several rather nasty chemicals (silver nitrate, and others), plus the silver tarnished rather too quickly, in a matter of months or less, and was delicate and easily damaged. So, the way it is done now, is to have it aluminized in a vacuum chamber. Aluminum lasts a lot longer, many years. Probably decades when protected inside the mirror housing of our Z cars.
One fellow that does such things, is named Royce.
He does not do the aluminizing himself, he subcontracts it to a lab, but you can get a referral to the company that aluminum coats his mirrors. It very well may be that they would perform this task for you, even though it is not the type and shape of mirror they are used to handling.
Astro mirror coatings are usually billed according to the diameter of a circular glass disk. I imagine they would need the approximate square centimeter or square inch area of the mirror to offer an estimate. But off hand, I can't think of a reason this would not work.