On a computer, I can use the OS to rotate the display on the monitor. The iPad does not seem to have this functionality within screen mirroring, and I was hoping to find it on the new stage manager functionality on the ipad pros, but I haven't figured that out either. That being said, a developer friend of mine took a look and said that Onsong has developed a setting to control screen orientation from within their app, as shown earlier.
If you can find a way to build this in, you will make a lot of people very happy. I keep seeing this feature request for iPads in general