> I can connect the the device to bluetooth, but it apparently doesn't detect
> it as a potential input device.

Bluetooth headsets typically offer multiple profiles, and usually Linux
defaults to the headphone (output only) profile.  The easiest thing I've
found is to install pavucontrol, because it puts everything necessary in
one panel.

There's a configuration tab - you can switch to a headset profile there,
then you can switch to the input tab to check the level.  And then when
you have an application trying to record, you can switch to the
recording and playback tabs to make sure it's using the correct devices.

Okay, that shows progress, but it still doesn't appear to present it as a possible input device.

The "Configuration" tab shows the BH-M9 headset, and I've selected "High Fidelity Playback (A2DP Sink: SBC)" as the option. The others are "unavailable."

However, the "Input Devices" tab shows "Built-In Audio Analog Stereo" with Port showing Front and Rear and Line In, all of which are "unplugged".