sorry to bump an old topic, but i just found an app today: Quantiloop Pro.
this is a 4 track looper that is AMAZING. you can save presets (different songs, store your loops), load in external tracks, tons of options for the actual looping functionality.
i even got BH to send MIDI commands to Quantiloop to change Presets and even Start playing, etc.
however there's a slight problem: even though i have "Let other apps play audio in background" turned on in Audio & MIDI, for some reason this particular app fades out and stops playing when i switch from it to BH. the app also puts the RED BAR on the top of iOS showing it's recording (even if it's just playing back) so perhaps it's a limitation of the app itself?
any insight on this one Arlo? this is almost PERFECT to use with just one iOS device - select a song in BH, it selects the preset in QL (this currently works), then record automation or use a BH button to play different loops/tracks as needed (the functions work, but audio isn't allowed).
for now i'm going to explore using 2 iOS devices - one for lyrics, then another for QL. i found a free app called midimittr that will allow 2 iOS devices to link for MIDI control via Bluetooth :O