Hi Dave,
I actually managed to hook up my sx900 and p121 with the app. I can use the p121 as controller for the SX.

Midi out sx connected to BT 01
Midi in P121 connected to iPad via the camera adapter gizmo.

So even if I don’t end up using the app as an arranger, it may have saved me having to buy a midi interface to connect the two together.