For testing, I added an override environment variable in some C# code so I can set it in the projects debug properties when I run it.

Turns out that while it's possible to do this in C++ projects, MS decided that they don't want you to do that for C# projects as there are other ways that they want to do it.

So it's not possible to modify your environment for C# projects within visual studio.

*Bangs head against the wall and surrenders to the Microsoft way of doing things*