BT working here with an old BT USB dongle I found.
CoreELEC:~ # dmesg
[26895.830409@3] usb 1-1: new full-speed USB device number 5 using xhci-hcd
[26895.982004@3] usb 1-1: New USB device found, idVendor=0c10, idProduct=0000, bcdDevice=15.00
[26895.982027@3] usb 1-1: New USB device strings: Mfr=1, Product=2, SerialNumber=3
[26895.982034@3] usb 1-1: Product: SiW
[26895.982040@3] usb 1-1: Manufacturer: SiW
[26895.982047@3] usb 1-1: SerialNumber: 1B3A05F61100
[26896.136776@3] Bluetooth: rfcomm_init_ttys: RFCOMM TTY layer initialized
[26896.136881@3] Bluetooth: rfcomm_init_sockets: RFCOMM socket layer initialized
[26896.136986@3] Bluetooth: rfcomm_init: RFCOMM ver 1.11
[26905.052701@0] input: MAXTEK AUDIO (AVRCP) as /devices/virtual/input/input12
Listening as I write on Maxtek Headphones.
EDIT: I found 4 different BT adapters and two worked and two did not.
One - a Broadcom - reported as unsupported.