Using OnSong and Sunday Keys for MainStage

You can use OnSong and Sunday Keys! OnSong provides more documentation on their end to help you set this up but here is a brief explanation for you: 

In OnSong, you can make it send a program change MIDI message. In MainStage, you will need to match that program change MIDI message from OnSong with the program change message of your patch. If you select a patch and go to the Attributes tab in the inspector, you can designate a program change number for that patch. Again, if you need more assistance, reach out to OnSong directly.