@peter this night not be the easiest thing to do. You can programmatically control the screen brightness on iOS see here: https://stackoverflow.com/questions/8936999/how-can-we-programmatically-change-the-brightness-of-the-iphone-screen
you could make a very simple iOS app that runs as a background service and accepts an OSC message from isadora. OpenFrameworks comes with an osc example for iOS, if you add the needed objective c code from page above you are almost there. You just need background activity.