This is one of those thigns where trivial and easy solution does not work, and in the process you find out that the philosophy is totally different from what you thought. Finally finished and published.
Temporary measures of today tend to become the legacy of tomorrow.
It looks like you can add a .csproj file as an item to your Visual Studio project, and it will be executed during the project build. I.e. Main.csproj may contain foo.cs, bar.cs, and custom.csproj as items, and every time you build Main.csproj it will invoke whatever build targets are in custom.csproj. Weird…
I found this by looking at someone else’s project that generated some source code files. I was trying to find where the custom build step for generating the files is. And there it was – inside a “nested” .csproj file, clearly written by hand.