Ah, with further explanation it makes sense , and not something that I have tried. I do have a portable Volumio device that I take in the car, but in that case I use the Volumio hotspot (and the sd card).
It would make a good guide when you get it all working. Good luck.
I’ve managed to get my Nexus 5x to create a network-over-bluetooth connection to the Pi. It’s not automatic yet and a lot can be improved but it’s another step.
However, I’m not a developer and although I know all the technologies that have been applied in Volumio by name (nice technical overview, btw!)
The raspbian image seems to be missing quite a bit of the bluetooth tools that are even in Debian Jessie proper.
So steps still to take:
upgrade both the kernel and the bluetooth toolchain to later versions to fully support Bluetooth LE.
automate the network detection with further systemd changes, including setting up DHCP on the bridged bnep interface.
make the volumio web interface listen on the bnep bridge address.
set up and automate routing if the wifi of the pi is connected to an access point? (Seems pointless, though, but cool.)
Redirect the volumio app/interface to the bluetooth provided IP address instead (weighted? forced? Might be a fixed address one can change by hand in a config file if needed.)
…
package into a plugin
I’m starting to think, though that this is best served with waiting for a raspbian (and therefore Volumio, I hope) upgrade to Debian stretch so that Raspbian isn’t really needed anymore.
Big question: is there a way to deploy the Volumio components ‘clean’ onto Debian stretch? That would simplify further development massively. There’s now a way to run Debian Stretch proper on Raspberry Pi so that would help.
This apparently simple suggestion actually turns into something far more complex than I was expecting. Updating the kernel is very much in the pipeline, and is hoped to be sooner than later. However, wholesale movement to Debian Stretch is not on the radar (at least as far as I am aware) for RPi.