TNServerInstance.Start() will have access to your game classes as you're running it from within your game.
I hadn't been able to get it to work within my game, I could only get it to work by putting the class file in the server.exe solution and rebuilding like you suggested. Today I dug into why, and it turns out it wasn't that the class wasn't available, that was just the message sent back, the real problem came from UnityTools.GetTypeEx(string). On line 294 it's doing an Application.IsPlaying check, and that is throwing an exception:
get_isPlaying can only be called from the main thread.
Constructors and field initializers will be executed from the loading thread when loading a scene.
Don't use this function in the constructor or field initializers, instead move initialization code to the Awake or Start function.
UnityEngine.Application:get_isPlaying()
By commenting out that if, I was able to get it to work. Maybe that check shouldn't be there?