I wanted to do the same thing and have a workaround that's been successful so far. It may work for you depending on what you want to run on the main screen.
I have two monitors, the first is a 21.5" touch screen set at 1920x1080 that runs another program; the second is a 21.5" monitor set at 1280x720 that runs the SignagePlayer.
I set up a screen template with size 3200x720 with a single screen division size 1280x720 aligned with the right edge of the screen template. My content plays only on the second monitor. However the SignagePlayer will try to take control of all the screens. I designed the program on the main touch screen monitor so I have commands in the software to move it to the top of the window stack and it plays over the SignagePLayer.
This will probably work for a lot of programs for your main monitor since most programs default to opening on top.
Best of luck,
Doug