I don't understand is this for bluetooth or for usb-wired headphones or both?

[iconoclasthero]

I have several bluetooth audio devices i would be interested in getting battery info on.

[Sapd]

Bluetooth is also HID, so in theory it should be possible to implement a device. Here is an tutorial for WireShark: https://github.com/Sapd/HeadsetControl/wiki/Development