Selfie Stick – Bluetooth HID Device on Windows Phone 8.1

It's perfectly possible. Just start a bluetooth server registered with the HID service Guid {00001124-0000-1000-8000-00805f9b34fb}. If the device supports the Microsoft bluetooth stack you can use Peter Foot's excellent .NET CF library (http://32feet.net/) and BluetoothService.HumanInterfaceDevice;

UPDATE:

With Peter Foot's library the server would look something like this:

using System.IO;
using InTheHand.Net.Sockets;
using InTheHand.Net.Bluetooth;

// ...

BluetoothListener l = new BluetoothListener(
    BluetoothService.HumanInterfaceDevice);
using (l) {
    BluetoothClient c = l.AcceptBluetoothClient();
    using (c) {
        Stream s = c.GetStream();
        using (s) {
            // send HID bytes
        }
    }
}

Regards, tamberg

It is definitely possible on Linux. Some time ago I found this project:

http://nohands.sourceforge.net/index.html

They emulate a full-blown headset with audio and keyboard controls on the Linux bluetooth stack. If they can emulate something like that, you would probably be able to emulate something simpler like a keyboard.