is there a chance I can get the pi to accept bluetooth audio streams from an iphone?
Why not use AirPlay instead?
Go into your PMB settings and enable AirPlay.
Then play music on your phone, pull up the Control Center and to the right of AirDrop you’ll see an option for AirPlay. Tap on it and select MusicBox (or whatever the hostname of your Pi is).
Note: Your PMB and your iPhone need to be in the same Wifi Network for this to work.