Hi Peter,
The app seems very useful, however I want to tell you about my experience:
with my band I use Stage Traxx for the backing tracks and, in the lyrics screen, I inserted the midi commands which (via Midimittr) activate a Mac computer with QLAB. With QLAB I can activate videos, logos, lights, texts, loops..etc.. in short I can do everything without problems, and even project on multiple screens or LED walls.
If your need is for an app, then QLAB is not good (it doesn't work on iPad). However, if you can bring a PC on stage, then QLAB is the best choice you can make.