To be fair, this is a very valid use case. I've wanted this right from the start since if you use external keyboard and mouse, you won't need the virtual one so the screen will be useless. In Samsung DeX, you can do this although I noticed that some apps wakes it up when doing certain things.
In any case, there's no way to do this right now. It might need changes to the lower level components such as Mir. Also note that unlike Samsung DeX, UT is using the same UI or DE when connected to an external display so anything you do on the device would affect both displays.
