Granted, I didn't modify Unity Engine, but I have enough experience with GTK 3.0 and QT to say that the button is only a means to fire an event.

This can be done with any other button and mouse wheels really aren't special in this regard. The spinning wheel simply causes a quick succession of wheel-up or wheel-down signals. The game engine has an event loop listener, that executes actions depeding on the input signal.

Besides, Unity Engine also has Cities:Skylines, which was made by 12 people and supported rebinding zoom-in and zoom-out camera controls from day one. I swapped mouse wheel for keyboard wihtout issue there, so it's not impossible.

Looking into its documentation[docs.unity3d.com], Unity seems to support mouse, keyboard and controller input by default. Though they built a SrollHandler class into the engine to handle scroll wheel input. It should be possible to convert the scroll wheel movement into Unity input type.
This article[www.programmersought.com] describes it in some detail. I may look into this more in-depth, but I'd need a way to send the findings to the developers.

Still, I'll probably have to recompile the dll in the short to medium term.