IOS and audio over usb

There is good and bad news here. Using an Ipad air 2 and a simple lightning camera adapter connected to a dgx670 digital piano, Pianoteq plays mixable audio through the piano's internal speakers. The downside is There is no way to reroute the audio, in particular to bluetooth headphones. To be clear, there seems to be no way to use the usb for  midi without audio sent over the same cable.

I have reported this as a bug but have not gotten a response. I suppose IOS is the biggest part of the problem, since no matter what I try, even if already connected and listening to a bluetooth stream, IOS takes over and routes the audio as described. Does anyone know some way around this problem?