Does UNet allow that kind of functionality where the server just acts as an intemediary to update clients rather than being the full instance of the game?
Well in theory you could do this. In your installation of unity you'll find the .dll files that make up UNET, which you could import into a fresh windows application and roll your own. I'm not sure there would be much benefit from doing this though?
Wooow, thanks for suggestion, seems like it is possible to implement some kind of light weight server with only updating your custom single .dll library which is contains all your network code (Server and Client), this library should contain registered commands and rpcs which is required to be registered on server, so I think adding the library will update the server mechanics also.