I've just tested a few things. I had to press play on a song in iReal Pro (v6.0.1) initially, then after that everything worked in AudioBus with a reverb as effects to a recording app so you might not have much of a problem if you want to do something with an effects app. I also tried activating Play in the recording app in iReal Pro, then the play button to start the song and this worked well.
Glad you can convert to MP3.