It might be tricky to have a generic iPad app that interfaces with Mainstage since the performance view for a concert is customized for each concert. The iPad app would somehow have to access the concert layout and reproduce the view, and the MIDI mappings. That could be possible and would be quite useful.
However, you could probably use a program like MIDI Designer to do something similar. You'd have to recreate the UI for your concert in MIDI Designer, and have each control there send the expected MIDI control sequence to the corresponding control in Mainstage.